Category: Uncategorized
-
Membrane Transport: Protein, Examples, and Types
Continue ReadingMembrane Transport
o Concentration of ions inside and outside a cell are very different:
o Na+ is more plentiful outside the cell
o Cl- is more plentiful outside the cell
o Ca+2 and Mg+2 are more plentiful outside the cell
o K+ is more plentiful inside the cell
o A couple rules dictate the permeability of solutes and ions:
1) The smaller the molecule the more permeable it is.
2) the lower the polarity, the more permeable it is.
3) Lipid bilayers are impermeable to all ions and charged molecules, no matter how small they are.
o Cell membranes allow water and small nonpolar molecules (including steroids) to permeate by simple diffusion (dictated by the chemical concentration gradient).
o For molecules with a charge there is also an electrical gradient pointing in the direction that a positively charged particle will tend to move.
o The two gradients can be added to form a single electrochemical gradient.
o The electrochemical gradient for compound X points in the direction that particle X will tend to move.
o Heat and pressure also affect the way particles diffuse.
o In strict terms, diffusion occurs in the direction of decreasing free energy and in increasing universal entropy.
o The ion flow changes the voltage across the membrane.
o The membrane potential, thus altering the electrochemical driving forces for the transmembrane movement of all the other ions.
Examples of Membrane Transport
o If compounds X and Y are separated by an impermeable membrane, diffusion is stopped.
o However, if the molecules of X can wiggle their way across the membrane then diffusion is only slowed.
o Since the membrane slows the diffusion of X, but doesn’t stop it, the membrane is said to be semipermeable to compound X.
o Facilitated diffusion is also said to make the membrane semipermeable.
o Most of the diffusion of polar and charged molecules across a natural membrane takes place through incidental holes (sometimes called leakage channels) created by the irregular shapes of integral proteins.
o This is passive diffusion (down its electrochemical gradient).
Types of Membrane Transport Proteins
1) Carrier Proteins:
a) allows passage only to solute molecules that fit into the binding site on the protein. Bind their solutes with great specificity much like how enzymes do
2) Channel Proteins:
a) discriminates mainly on the basis of size and electric charge
b) Most of the channel proteins in the plasma membrane of animal and plant cells are quite different and have narrow, highly selective pores.
o Almost all of these proteins are ion channels.
o Concerned exclusively with the transport of inorganic ions, mainly Na+ , K+, Cl-, and Ca+2
o There are three types of ion channels:
i. Voltage gated ion channels: probability of being open is determined by the membrane potential.
o All living cells have gated sodium-potassium ATPases and/or other pumps that are voltage dependent
ii. Ligand gated ion channel: controlled by the binding of some molecule (the ligand) to the channel protein.
iii. Stress-activated channel: opening is controlled by a mechanical force applied to the channel.
o Ex. In your ear
o Ion channels are either fully open or fully closed.
Facilitated Diffusion
o Facilitated diffusion is diffusion in which the molecule cant naturally diffuse (because of its size, polarity or both) across the membrane so one of these membrane transport proteins assists it.
o It is still down the electro-chemical gradient however.
o Most, but not all, human cells rely on facilitated diffusion for their glucose supply.
o Only certain epithelial cells in the digestive tract and the proximal tube of the kidney are capable of absorbing glucose against a concentration gradient.
o This is done via a secondary active transport mechanism down the concentration gradient of sodium.
o In the absence of insulin, only neural and hepatic cells are capable of absorbing sufficient amounts of glucose via the facilitated transport system.
Passive Transport
o Facilitated diffusion is said to make the membrane selectively permeable because it is able to select between molecules of similar size and charge.
o To move a solute against its concentration gradient, a transport protein must do work: it has to drive the “uphill” flow by coupling it to some other process that provides energy (usually via ATP hydrolysis or through coupling it with a “downhill” movement). This is called active transport.
o It can also be accomplished indirectly by using ATP to create an electrochemical gradient, and then using the energy of the electrochemical gradient to acquire or expel a molecule.
o The latter method is called secondary active transport.
o Whenever we are moving against an electrochemical gradient it MUST be active transport
"Ion channels are either fully open or fully closed"
o For coupling transport of two molecules:
o If both solutes move in the same direction its called symport.
o If they move in opposite directions its called antiport.
o If a carrier protein ferries only one type of molecule across the membrane (and is therefore not a couple transporter) its called a uniport.
Ion Transporters
o Under normal conditions, the interior of most cells is at a negative electrical potential compared to the exterior, so that positive ions tend to be pulled into the cell.
o Thus the inward electrochemical driving force for Na+ is large, as it includes the driving force due to the concentration gradient and a driving force in the same direction to the voltage gradient.
o Therefore to maintain this balance we need a pump which pumps out Na+.
o This is achieved through the Na+-K+ pump.
o It couples the export of Na+ to the import of K+ and hydrolyzes ATP to do so.
o The Na+-K+ pump helps to maintain osmotic balance of pressure in a cell so it wont flood with H2O and burst.
Membrane Transport Citations
- Membrane transport in the cellular homeostasis of calcium. J Cardiovasc Pharmacol . 1986;8 Suppl 8:S3-6.
- Effect of DNA conformation on facilitated diffusion. Biochem Soc Trans . 2013 Apr;41(2):582-8.
- Facilitated diffusion of glucose. Physiol Rev . 1990 Oct;70(4):1135-76.
- Facilitated Diffusion Mechanisms in DNA Base Excision Repair and Transcriptional Activation. Chem Rev . 2018 Dec 12;118(23):11298-11323.
- Facilitated diffusion of Argonaute-mediated target search. RNA Biol . 2019 Sep;16(9):1093-1107.
- Host-microbe interactions via membrane transport systems. Environ Microbiol . 2015 Apr;17(4):931-7.
- Analysis of membrane transport mechanisms of endogenous substrates using chromatographic techniques. Biomed Chromatogr . 2019 Jun;33(6):e4495.
- Vitamin E intestinal absorption: Regulation of membrane transport across the enterocyte. IUBMB Life . 2019 Apr;71(4):416-423.
Share
Similar Post:
-
Cell Membrane / Plasma Membrane: Structure and...
Continue ReadingPlasma Membrane / Cell Membrane
o The cytosol of nearly all prokaryotes is surrounded by a phospholipid bilayer called the plasma membrane (the membranes of archaea differ in their lipid structure).
o It gives the cell its basic structure and serves as a permeability barrier.
o The phospholipid is composed of a phosphate group, two fatty acid chains, and a glycerol backbone.
o The phospholipid group is polar, while the fatty acid chains are nonpolar making the molecule amphipathic.
o When placed in aqueous solution, amphipathic molecules spontaneously aggregate, turning their polar ends toward the solution, and their nonpolar ends toward each other.
o The resulting spherical structure is called a micelle.
o If enough phospholipids exist, and the solution is subjected to ultrasonic vibrations, liposomes may form.
o A liposome is a vesicle surrounded and filled by aqueous solution.
o It contains a lipid bilayer like that of a plasma membrane.
o The inner and outer layers of a membrane are referred to as leaflets.
o The level of saturation in the fatty acids of the phospholipids also determines the membranes fluidity; an increase in the unsaturation of these fatty acids increases the fluidity of the membrane.
o If we increase the temperature as well the membrane fluidity increases.
o The plasma membrane contains other types of lipids such as glycolipids.
o Different lipid types are arranged asymmetrically between the leaflets.
o For instance, glycolipids are found on the outer leaflet only.
o Unlike eukaryotic membranes, prokaryotic membranes usually DON’T contain steroids such as cholesterol.
Instead, some bacterial membranes contain steroid like molecules called hopanoids.
o Cholesterol tends to stiffen the bilayer, making it more rigid and less permeable.
o Hoponoids probably reduce the fluidity of the membrane in Prokaryotes.
o In eukaryotes nearly all new membrane synthesis occurs in the ER.
o Also embedded within the plasma membrane are proteins.
o Most of the functional aspects of membranes are due to their proteins.
o Membrane proteins act as transporters, receptors, attachment sites, and enzymes.
Organization of Plasma Membrane
Types of Protein in Plasma Membrane
o Two types of proteins are involved with the plasma membrane:
1) Integral Proteins: Ampipathic proteins that transverse the membrane from inside the cell to the outside.
o Are usually ion channels.
2) Peripheral or Extrinsic Proteins: are situated entirely on the surfaces of the membrane.
o They are ionically bonded to integral proteins or the polar group of a lipid.
o Both integral and peripheral proteins may contain carbohydrate chains making them glycoproteins and the carbohydrate portion always protrudes toward the outside of the cell.
o Proteoglycans also exist on the membrane.
o It can thus form a carbohydrate layer (glycocalyx) which protects the cell surface from chemical and mechanical damage.
o Proteoglycans are also a mixture of proteins and carbohydrates, but they generally consist of more than 50% carbohydrates.
o Proteoglycans are a major component of extracellular matrix.
o Glycoproteins are proteins with a carbohydrate group attached and they are a component of cellular plasma membranes.
o Also serve as markers for cellular recognition.
o Lipoproteins also exist in some plasma membranes with their lipid portions embedded in the membrane and their protein portions at the surfaces.
o Membrane proteins are distributed asymmetrically throughout the membrane and between the leaflets.
o Neither proteins nor lipids easily flip from one leaflet to the other.
o The shape of the cell and the mechanical properties of the plasma membrane are determined by a meshwork of fibrous proteins, called the cell cortex, that is attached to the cytosolic surface of the membrane.
o It is made up primarily of spectrin.
o Since the forces holding the entire membrane are intermolecular, the membrane is fluid; its parts can move laterally but can’t separate.
o The model of the membrane as just described is known as the fluid mosaic model.
Plasma Membrane Citations
Share
Similar Post:
-
Prokaryotes: Prokaryotic Cells, Structure, Organization, and Facts
Continue ReadingWhat are Prokaryotes
o Prokaryotes don’t have a membrane bound nucleus.
o They are split into two distinct domains called Bacteria and Archaea.
o Archaea have as much in common with eukaryotes as they do with bacteria.
o They are typically found in the extreme environments such as salty lakes and boiling hot springs.
o Unlike bacteria, the cell walls of archaea are not made from peptidoglycan.
o Most known prokaryotes are members of the domain Bacteria.
o The introduction of the two domains makes the kingdom Monera obsolete.
o The kingdom Monera was the kingdom containing all prokaryotes.
o In order to grow; all organisms require the ability to acquire carbon, energy and electrons (usually from hydrogen).
o Organisms can be classified according to the sources from which they gather these commodities.
o A carbon source can be organic or inorganic.
o Most carbon sources also contribute oxygen and hydrogen.
o CO2 is a unique inorganic carbon source because it has no hydrogens.
o To some degree, all microorganisms are capable of fixing CO2 (reducing it and using the carbon to create organic molecules usually through a process called the Calvin Cycle).
"Autotrophs are organisms that are capable of using CO2, as their sole source of carbon"
"Heterotrophs use preformed, organic molecules as their source of carbon"
Classifications
o Organisms can be classified as well according to how they get their energy:
1) Phototrophs use light as their energy source.
o Ex. Cyanobacteria: blue/green algae that uses photosynthesis.
o Only prokaryotes can acquire energy from an inorganic source other than light.
2) Chemotrophs use the oxidation of organic and inorganic matter as their energy source.
o Electrons or hydrogens can be acquired from inorganic matter by lithotrophs, or organic matter by organotrophs.
o You can recall this one because lithium is an inorganic molecule and lithotrophs get their hydrogens from inorganic molecules.
o Humans are chemoheterotrophs.
All organisms can be classified as one of each of the three types.
o Bacteria are found in all classifications.
o Some bacteria are capable of fixing nitrogen.
o Atmospheric nitrogen is abundant, but in a strongly bound form that is useless to plants.
o Nitrogen fixation is the process by which N2 is converted to ammonia.
o Most plants are unable to use ammonia however and must wait for other bacteria to further process the nitrogen in a process called nitrification.
o Nitrification is a two step process that creates NITRATES, which are useful to plants, from ammonia.
o Nitrification requires two genera (genus’s) of chemoautotrophic prokaryotes.
o Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates.
o Degradation of ammonia to nitrite is usually the rate limiting step of nitrification.
o Genus – (plural: genera) taxonomic group containing one or more species.
o Chemoautotrophy is an inefficient mechanism for acquiring energy, so chemoautotrophs require large amounts of substrate.
o All known chemoautotrophs are prokaryotes.
o Only prokaryotes can acquire energy from an inorganic source other than light.
Structure of Prokaryotes
o Prokaryotes have a single, circular double stranded molecule of DNA.
o This molecule is twisted into supercoils and is associated with histones in Archaea and with proteins that are different from histones in bacteria.
o The DNA, RNA and protein complex in prokaryotes forms a structure visible under the light microscope called a nucleoid (also called the chromatin body, nuclear region, or nuclear body).
o The nucleoid is not enclosed by a membrane.
Structure of Bacteria
o There are three major shapes of bacteria:
1) cocci (round)
2) bacilli (rod shaped)
3) spiral
o There are other shapes, including helical.
o Helically shaped bacteria are called spirilla, if they are rigid. Otherwise they are called spirochetes.
o Certain species of spirochetes may have given rise to eukaryotic flagella through a symbiotic relationship.
o The name of the bacteria often reveals the shape.
o Prokaryotes have no complex, membrane-bound organelles.
o The key words are ‘complex’ and ‘membrane bound’.
o They have organelles: ribosomes, nucleoids, and mesosomes etc….; just not complex membrane-bound organelles like mitochondria, ER, Golgi, etc.
o Prokaryotic ribosomes are smaller than eukaryotic ribosomes.
o They are made from a 30S subunit and a 50S subunit to form a 70S subunit.
o A prokaryote may or may not contain a mesosome.
o Mesosomes are invaginations of the plasma membrane and can be seen under the light microscope.
o Their exact function is unknown, but may be involved in cell wall formation during cell division.
o Prokaryotes also have inclusion bodies.
o Inclusion bodies are granules of organic or inorganic matter that may be visible under a light microscope.
o Inclusion bodies may or may not have a membrane.
Prokaryotes Citations
- Predation between prokaryotes and the origin of eukaryotes. Bioessays . 2009 Jul;31(7):748-57.
- From Prokaryotes to Cancer: Glutamine Flux in Multicellular Units. Trends Endocrinol Metab . 2017 Sep;28(9):637-644.
- Glycoproteins in prokaryotes. Arch Microbiol . 1997 Sep;168(3):169-75.
- Prokaryotes: the unseen majority. Proc Natl Acad Sci U S A . 1998 Jun 9;95(12):6578-83.
Share
Similar Post:
-
Viruses: Structure, Function, and Facts
Continue ReadingWhat are Viruses
o Viruses are tiny infectious agents, much smaller than bacteria.
o They are comparable in size to large proteins.
o In its most basic form, a virus consists of a protein coat, called a capsid, and from one to several hundred genes in the form of DNA or RNA inside the capsid.
o No virus contains both DNA and RNA.
o Most animal viruses, some plant viruses, and very few bacterial viruses surround themselves with a lipid-rich envelope either borrowed from the membrane of their host cell or synthesized in the host cell cytoplasm.
o The envelope typically contains some virus-specific proteins.
o The capsid usually accounts for most of the weight of the virus.
o A mature virus outside the host cell is called a virion.
Viruses Types and Structures
o Viruses are not currently classified as living organisms; they do not belong to any of the taxonomical kingdoms of organisms.
o Viruses differ from living organisms in the following ways.
o They ALWAYS require the host cell’s reproductive machinery in order to replicate.
o Viruses don’t metabolize organic nutrients.
o Instead they use the ATP made available by the host cell.
o Viruses possess either RNA or DNA, but never both.
o Thus, there are viruses with the familiar double-stranded DNA, with single-stranded DNA, with double-stranded RNA, and with single-stranded RNA.
o Also the nucleic acid could be linear or circular.
o Viruses can be crystallized without losing their ability to infect.
o A viral infection begins when a virus adsorbs to a specific chemical receptor site on the host.
o The host is the cell being infected.
o The chemical receptor is usually a specific glycoprotein on the host cell membrane.
Life Cycle of Viruses
o The virus can’t infect the cell if the specific receptor isn’t available/there.
o Next, the nucleic acid of the virus penetrates into the cell.
o In a bacteriophage, a virus that infects bacteria, the nucleic acid is normally injected through the tail after viral enzymes have digested a hole in the cell wall.
o Most viruses that infect eukaryotes are engulfed by an endocytotic process.
o They could also be engulfed by a process called membrane fusion, in which the envelope of the virus is actually incorporated into the plasma membrane of the host cell releasing the capsid into the cytoplasm.
Types of Viruses Infection
Once inside the cell, there are two possible paths;
I. Lytic infection
o The virus commandeers the cell’s reproductive machinery and begins reproducing new viruses.
o There is a brief period before the first fully formed virion appears.
o This period is called the eclipse period.
o The cell may fill with viruses until it lyses or burst, or it may release the new viruses one at a time in a reverse endocytotic process.
o The period of infection to the lysis is called the latent period.
o The latent period encompasses the eclipse period.
o A virus following the lytic cycle is called a virulent virus.
II. Lysogenic infection
o The viral DNA is incorporated into the host genome, or, if the virus is an RNA virus and it possesses the enzyme reverse transcriptase.
o DNA is actually reverse-transcribed from RNA and then incorporated into the host cell genome.
o When a host replicates its DNA, the viral DNA is replicated as well.
o A virus in a lysogenic cycle is called a temperate virus.
"The chemical receptor is usually a specific glycoprotein on the host cell membrane"
o While the viral DNA remains incorporated in the host cell, the virus is said to be dormant or latent, and is called a provirus (a prophage if the host cell is a bacterium).
o There are 2 important results from the lysogenic cycle:
1) The cell infected is immune to reinfection by the same phage.
2) The host cell may exhibit new properties, this is known as phage conversion.
o The dormant virus may become active when the host cell is under some type of stress.
o Examples of stress include UV light or carcinogens.
o When the virus becomes active, it becomes virulent.
o Exhibits exponential growth b/c each new cell will create more viruses.
o It is a longer cycle than the lytic cycle.
Type of Viruses
o There are many types of viruses.
o One way to classify them is by the type of nucleic acid that they posses.
o A virus with unenveloped plus-strand RNA is responsible for the common cold.
o The “plus-strand” indicates the proteins can be directly translated from the RNA.
o Enveloped plus-strand RNA viruses include retroviruses such as the virus that causes AIDS.
o A retrovirus carries the enzyme reverse transcriptase in order to create DNA from its RNA.
o The DNA is then incorporated into the genome of the host cell.
o Minus-strand RNA viruses include measles, rabies, and the flu.
o Minus- strand RNA is the complement to mRNA and must be transcribed to plus-RNA before being translated.
o There are even double stranded RNA viruses, and single and double stranded DNA viruses.
o Reassortment – is a method with which virus’ can alter their genetic makeup it occurs If a virus has a segmented genome and if two variants of that virus infect a single cell, progeny virions can result with some segments from one parent, some from the other.
o Viroids are a related form of infectious agent.
o Viroids are small rings of naked RNA without capsids.
o Viroids only infect plants.
o There also exist naked proteins called prions that cause infections in animals.
o Prions are capable of reproducing themselves apparently without DNA or RNA.
Defense Against Viral Infection
o Although the lipid rich envelope is borrowed from the host cell, spike proteins encoded from the viral nucleic acids protrude from the envelope.
o These proteins bind to receptors on a new host cell causing the virus to be infectious.
o However, it is also the spike proteins that human antibodies recognize when fighting the infection.
o Since RNA polymerase doesn’t contain a proofreading mechanism, changes in the spike proteins are common in RNA viruses.
o When the spike proteins change, the antibodies fail to recognize them, and the virus may avoid detection until new antibodies are formed.
o A vaccine can be either an injection of antibodies or an injection of a nonpathogenic virus with the same capsid or envelope.
o The later allows the host immune system to create its own antibodies.
o Vaccines against rapidly mutating viruses are generally not very effective.
o Another difficulty of fighting viral infections is that more than one animal may act as a carrier population.
o Even if all viral infections of a certain type were eliminated in humans, the virus may continue to thrive in another animal, thus maintaining the ability to reinfect the human population.
o For instance, ducks carry the flu virus, apparently without any adverse symptoms.
o One of the reasons that the fight against smallpox was so successful was because the virus can only infect humans.
"The structure of a virus: capsid, nucleic acid, and lipid-rich protein envelope for some viruses: tail, base plate, and tail fibers for most bacteriophages"
Viruses Citations
Share
Similar Post:
-
Meiosis: Phases, Diagram, Stage, and Checkpoints
Continue ReadingWhat is Meiosis?
o Meiosis is double nuclear division which produces four haploid gametes (also called germ cells).
o In humans, only the spermatogonium and the oogonium (both diploid) undergo meiosis.
o All other cells are somatic and undergo mitosis.
o The gametes are haploid.
o After replication occurs in S phase of interphase, the cell is called a primary spermatocyte or primary oocyte (both diploid).
o In the human female, replication takes place before birth, and the life cycle of all germ cells are arrested at the primary oocyte stage until puberty.
o Arrested in prophase 1 Just before ovulation, a primary oocyte undergoes the first meiotic division to become a secondary oocyte (haploid).
o The secondary oocyte is released upon ovulation, and the penetration of the secondary oocyte by the sperm stimulates anaphase II of the second meiotic division in the oocyte.
o Meiosis is two rounds of division called meiosis I and meiosis II.
o Meiosis I proceeds similarly to mitosis with the following differences.
Diagram of Meiosis Phases
Stages of Meiosis
Prophase I
In prophase I homologous chromosomes line up along side each other, matching there genes exactly.
o At this time, they may exchange sequences of DNA nucleotides in a process called crossing over (synapsis).
o Genetic recombination in eukaryotes occurs during crossing over.
o You can also have Double crossovers:
Scenario 1: results in no genetic recombination. The chromatids involved in this double crossover exchange alleles at first, but then it exchanges them back, resulting in no net recombination. This is called the 2-strand double crossover. Results in 0/4 recombinants.
o Scenario 2: results in genetic recombination. The chromatids exchange alleles during a crossover. Then, one of the crossover chromatid exchanges with a different chromatid. This is called the 3-strand double crossover. Results in 2/4 recombinants.
o Scenario 3: results in genetic recombination. The chromatids exchange, then 2 totally different chromatids on the same chromosome exchange. This is called the 4-strand double crossover. Results in 4/4 recombinants.
o Since each duplicated chromosome in prophase I appear as an ‘x’, the side by side homologues exhibit a total of four chromatids, and are called tetrads.
o If crossing over does occur, the two chromosomes are “zipped” along each other where nucleotides are exchanged, and form what is called the synaptonemal complex.
o A chiasma (plural: chiasmata) is thought to be the point where two homologous non-sister chromatids exchange genetic material during chromosomal crossover during meiosis.
o Sister chromatids also form chiasmata between each other, but because their genetic material is identical, it does not cause any change in the resulting daughter cells.
o Genes located close together on a chromosome are more likely to cross over together, and are said to be linked!!!
Metaphase I
o In metaphase I the homologues remained attached, and move to the metaphase plate.
o Rather than single chromosomes aligned along the plate as in mitosis, tetrads align in meiosis.
Anaphase I
o Anaphase I separates the homologues from their partners, the centromeres stay together (which is different from anaphase in mitosis).
Telophase I
o In telophase I, a nuclear membrane may or may not reform, and cytokinesis may or may not occur. In humans the nuclear membrane does reform and cytokinesis does occur.
o If cytokinesis occurs, the new cells are haploid with 23 replicated chromosomes, and are called secondary spermatocytes and secondary oocytes.
o In the case of a female, one of the oocytes, called the first polar body, is much smaller and degenerates (it may or may not go through meiosis II).
o This occurs in order to conserve cytoplasm, which is contributed only by the ovum.
"These four phases together are called meiosis I. Meiosis I is reduction division"
o Meiosis II proceeds with prophase II, metaphase II, anaphase II, and telophase II.
o The final products are haploid gametes each with 23 chromosomes.
o In the case of the spermatocytes, four sperm cells are formed. In the case of the oocyte, a single ovum is formed.
o In the female, telophase II produces one gamete and a second polar body.
o If during anaphase I or II the centromere of any chromosome doesn’t split, this is called nondisjunction (it can also happen in mitosis but the ramifications are less severe).
o As a result of primary nondisjunction (nondisjunction in anaphase I), one of the cells with end up with two extra chromatids
o Complete extra chromosome) and the other will be missing a chromosome.
o If nondisjuction occurs in anaphase II that will result in one cell having an extra chromatid and one lacking a chromatid.
o The number of different possible gametes that can be formed by diploid organisms as a result of independent assortment of chromosomes during meiosis can be calculated by using the formula 2^n where n is the number of heterozygous genes.
o Ex. AaBbCc ⇒ this can produce 2^3 number of different haploid cells.
o Parthenogenesis (means the growth and development of an embryo or seed without fertilization by a male.
o Parthenogenesis occurs naturally in some lower plants, invertebrates (e.g. water fleas, aphids) and some vertebrates (e.g. lizards, salamanders, some fish, and even turkeys).
o Parthenogenetic populations are typically all-female.
o As with all types of asexual reproduction, there are both costs and benefits associated with parthenogenesis.
o Spermatogonium (diploid) ⇒ primary spermatocyte (diploid) ⇒ 2ndary spermatocyte (haploid) ⇒ spermatid (haploid) ⇒ spermatozoa (haploid)
o Oogonium (diploid) ⇒ primary oocyte (diploid) ⇒ secondary oocyte (haploid) ⇒ zygote (diploid)
o A Barr body is a permanently inactivated X chromosome that forms a dense stainable nuclear mass.
o A normal female with XX inactivates one of her X’s while expressing the other.
o She therefore has 1 Barr body.
o A normal male, who is XY, does not inactivate his only X chromosome, and therefore has no Barr bodies.
o The rule is that the number of X chromosomes is always 1 MORE than the number of Barr bodies.
o Ex. A person with 2 Barr bodies has to have 3 X chromosomes.
Meiosis Citations
- Elevated Mutagenicity in Meiosis and Its Mechanism. Bioessays . 2019 Apr;41(4):e1800235.
- Recombination, Pairing, and Synapsis of Homologs during Meiosis. Cold Spring Harb Perspect Biol . 2015 May 18;7(6):a016626.
- Meiosis: the chromosomal foundation of reproduction. Biol Reprod . 2018 Jul 1;99(1):112-126.
Share
Similar Post:
-
Mitosis: Phases, Diagram, Stage, and Checkpoints
Continue ReadingWhat is Mitosis
o Mitosis is nuclear division without genetic change, and results in genetically identical daughter cells,
o Mitosis varies among eukaryotes.
o For instance, fungi don’t have centrioles (prokaryotes and plants don’t either) and never lose their nuclear membranes.
o Mitosis occurs in both haploid and diploid cells.
o Prokaryotes don’t undergo mitosis or meiosis!!
Diagram of Mitosis Phases
Stages of Mitosis
i. Prophase
o Condensation of chromatin into chromosomes occurs.
o Centrioles move to opposite ends of the cells.
o First the nucleolus and then the nucleus disappear.
o The spindle apparatus begins to form consisting of aster (microtubules radiating from the centrioles), kinetochore microtubules growing from the centromeres (a group of proteins located toward the center of the chromosome), and spindle microtubules connecting the two centrioles.
o The kinetochore is a structure of protein and DNA located at the centromere of the joined chromatids of each chromosome.
o The centromere allows one copy of each chromatid to be apportioned to each daughter cell
ii. Metaphase
o Chromosomes line up along the equator of the cell.
3) Anaphase – begins when the sister chromatids split at their attaching centromeres, and move toward opposite ends of the cell. This split is termed disjunction!!! Cytokinesis, the actual separation of the cellular cytoplasm due to constriction of microfilaments about the center for the cell, may commence toward the end of this phase. 4) Telophase – The nuclear membrane reforms followed by the reformation of the nucleolus. Chromosomes decondense and cytokinesis continues.
Remember that mitosis results in genetically identical daughter cells since its asexual reproduction!!
iii. Anaphase
o Anaphase begins when the sister chromatids split at their attaching centromeres, and move toward opposite ends of the cell.
o This split is termed disjunction.
o Cytokinesis, the actual separation of the cellular cytoplasm due to constriction of microfilaments about the center for the cell, may commence toward the end of this phase.
4) Telophase – The nuclear membrane reforms followed by the reformation of the nucleolus. Chromosomes decondense and cytokinesis continues.
Remember that mitosis results in genetically identical daughter cells since its asexual reproduction!!
iv. Telophase
o The nuclear membrane reforms followed by the reformation of the nucleolus.
o Chromosomes decondense and cytokinesis continues.
o Remember that mitosis results in genetically identical daughter cells since its asexual reproduction.
Mitosis Citations
Share
Similar Post:
-
Cell Cycle: Phases, Diagram, Stage, and Checkpoints
Continue ReadingCell Cycle Phases
o Every cell has a life cycle that begins with the birth of the cell and ends with the death or division of the cell.
Cell Cycle Diagram
o M ⇒ Mitosis
o G1 ⇒ Gap 1
o G2 ⇒ Gap 2
o S ⇒ Synthesis
o G0 ⇒ Gap 0/Resting.
Quiescent/ Senescent
Gap 0 / G0
o The cell has left the cycle and has stopped dividing.
o In humans, liver cells spend a great deal of time in G0.
o Mature neurons and muscle cells are in G0 permanently.
o Liver (hepatic) and pancreatic cells are normally in G0 but can reenter normal division
Interphase
Gap / G1 Phase
o G1 is normally the longest phase.
o Cells increase in size in Gap 1, regions of heterochromatin has been unwound and decondensed.
o RNA synthesis and protein synthesis is very active.
o The G1 checkpoint control mechanism ensures that everything is ready for DNA synthesis.
o The main factor in triggering the beginning of S is cell size based upon the ratio of cytoplasm to DNA.
Cell Cycle Checkpoints
Synthesis / S Phase
o DNA replication occurs during this phase.
o Organelles and proteins are produced more slowly.
o Each chromosome is exactly duplicated, but the cell is still considered to have the same number of chromosomes, only now, each chromosome is made of two identical sister chromatids.
Gap 2 / G2 Phase
o During the gap between DNA synthesis and mitosis, the cell will continue to grow.
o Cellular organelles continue to duplicate.
o RNA and protein (especially tubulin for microtubules) are actively synthesized.
o The G2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis/meiosis) phase and divide.
o It checks for mitosis promoting factor (MPF), when the level is high enough, mitosis is triggered.
Cell Division / Mitosis / Meiosis / M Phase
o Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter cells.
o A checkpoint in the middle of mitosis (Metaphase Checkpoint) ensures that the cell is ready to complete cell division.
Cell Cycle Citations
Share
Similar Post:
-
Chromosomes, Chromatin, Euchromatin, Heterochromatin
Continue ReadingChromosomes
o Sections of DNA that aren’t in use are wrapped tightly around globular proteins called histones.
o Eight histones wrapped in DNA form a nucleosome.
o Nucleosomes, in turn, wrap into coils called solenoids, which wrap into supercoils.
o The entire DNA/protein complex (including a small amount of RNA) is called chromatin.
o The basicity of histones gives them a net positive charge at the normal pH of a cell.
Types of Chromatin
Chromatin is found in two varieties: euchromatin and heterochromatin.
Euchromatin
o Euchromatin is chromatin that can be uncoiled and transcribed.
o Euchromatin is only coiled during nuclear division.
Heterochromatin
o Heterochromatin is a tightly packed form of DNA.
o Its major characteristic is that transcription is limited.
o Some chromatin called constitutive heterochromatin, is permanently coiled.
Chromosomes Structure
o In the nucleus of a human somatic cell, there are 46 double stranded DNA molecules.
o The chromatin associated with each one of these molecules is called a chromosome.
o In human cells, each chromosome possesses a partner that codes for the same traits as itself.
o Two such chromosomes are called homologues.
o Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis.
o The pair are non-identical chromosomes that both contain information for the same biological features and contain the same genes at the same loci but possibly each have different alleles (that is, different genetic information) at those genes.
o Any cell that contains homologous chromosomes are called diploid.
o Any cell that doesn’t contain homologous chromosomes are called haploid.
o In the nucleus of human cells, there are 46 chromosomes before replication, and 46 chromosomes after replication.
o The duplicates can be referred to separately as sister chromatids.
"Prokaryotic chromosomes exist in the cytoplasm"
o Eukaryotes have multiple pairs of linear chromosomes.
o The gene-coding sequences are interspersed with non-coding regions that may control gene regulation.
o Within a gene sequence, there are exons (coding regions) and introns (non-coding regions that will be removed from the mRNA).
o Eukaryotic chromosomes are contained in a nucleus.
o Prokaryotes have 1 circular chromosome, and may have smaller extrachromasomal DNA in the form of plasmids.
o Prokaryotic DNA does not have the non-coding regions or introns.
o Bacteria do NOT have histones, but archaea have histone-like structures associated with their chromosomes.
o Prokaryotic chromosomes exist in the cytoplasm.
o Reality about Bacterial Chromosomes: Although many have only the 1 circular chromosome, there are many exceptions to this rule.
Chromosomes Citations
Share
Similar Post:
-
Cancer: Types, Symptoms, Causes, and Tests
Continue ReadingCancer Cells
o Cancer cells continue to grow and divide indefinitely.
o A mass of cancer cells is called a tumor.
o A tumor is benign if it is localized in a small lump.
o When an individual has a tumor invasive enough to impair function of an organ, the tumor is said to be malignant.
o Cancer cells may separate from the tumor and enter the body’s circulatory systems and establish tumors in other parts of the body.
o This process is called metastasis.
o Certain genes that stimulate normal growth in human cells are called proto- oncogenes.
o Proto-oncogenes can be converted to oncogenes genes that cause cancer, by mutagens such as UV radiation, chemicals, or simply by random mutations.
o Mutagens that can cause cancer are called carcinogens.
o If and when DNA damage is discovered, the cell will immediately activate its tumor suppressor genes that halt the cell cycle in an attempt to repair the genetic lesion.
o If the genetic lesion is too extensive, an intracellular auto-death pathway known as apoptosis, will be activated and cell self-digestion would follow.
o The attempt to carry out apoptosis may fail, perhaps because of a preexisting defect in the repair mechanism, which would increase the possibility for malignancy to develop.
Citations
Share
Similar Post:
-
Mutations: Definition, Types, Causes, and Facts
Continue ReadingMutations Definition
o Any alteration in the genome that is not genetic recombination is called a mutation.
o A gene mutation is the alteration in the sequence of DNA nucleotides in a single gene.
o A chromosomal mutation occurs when the structure of a chromosome is changed.
o Gene mutation rates vary depending on size of gene and differences in susceptibility of particular genes to various mechanisms that cause mutations.
o A mutation in a somatic cell is unlikely to have drastic effects, but a mutation in a germ cell, from which all other cells arise, can be very serious for the offspring.
o Mutations can be spontaneous or they can be induced (occurring due to physical or chemical agents called mutagens).
o A mutagen is any physical or chemical agent that increases the frequency of mutation above the frequency of spontaneous mutations.
Types of Mutations
o If a mutation changes a single base-pair of nucleotides in a double strand of DNA, that mutation is called a point mutation.
o One type of point mutation is called a base-pair substitution results when one base-pair is replaced by another.
Schematic Representation of Point Mutations
o A base pair mutation involving a complete switch from A-T base-pair to the G-C base pair or the opposite is called a transition mutation.
o A base-pair substitution that involves a reversal of the same base-pairs is called a transversion mutation.
o Mnemonic: “In transversion mutations there is a conversion of purine ↔ pyramadine”
o A missense mutation is a base-pair mutation that occurs in the amino acid coding sequence of a gene.
o If there is no change in protein function, the mutation is called a neutral mutation, and if the amino acid is not changed, it is called a silent mutation.
o Even a silent mutation may be significant b/c it my change the rate of transcription.
o Ex. Deamination: Spontaneous loss of C and turns to U
Schematic Representation of Frameshift Mutations
o A second type of point mutation, an insertion or deletion of a base-pair, may result in a frameshift mutation.
o A frameshift mutation results when the deletions or insertions occur in multiples other than 3.
o A nonframeshift mutation occurs when a multiple of 3 is added or deleted.
o Frameshift mutations often result in a completely nonfunctional protein, whereas non-frameshift mutations may still result in a partially or even completely active protein.
o Base pair mutations are nonframeshift mutations as well.
o Ex. Depurinination – A&G are lost (resembles missing teeth)
o If a base-pair substitution or insertion or deletion mutation creates a stop codon, a nonsense mutation results.
o Nonsense mutations are very serious for the cell because they prevent the translation of a functional protein entirely.
o Mnemonic: “Stop the Nonsense”
Chromosomal Deletion
Chromosomal deletions occur when a portion of the chromosome breaks off, or when a portion of the chromosome is lost during homologous recombination and/or crossing over events.
Chromosomal Duplication
Chromosomal duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.
o The second copy of the gene is often free from selective pressure — that is, mutations of it have no deleterious effects to its host organism.
o Thus it mutates faster than a functional single-copy gene, over generations of organisms.
o Deletion/Duplication can occur with entire chromosomes (aneuploidy) or even sets of entire chromosomes (polyploidy).
DNA Recombination
o Homologous recombination is a type of genetic recombination in which genetic material is exchanged between two similar or identical strands of DNA.
o Although most widely used in cells to accurately repair double-strand breaks in DNA, homologous recombination also produces new combinations of DNA sequences during chromosomal crossover in meiosis.
o Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis.
o The pair are non-identical chromosomes that both contain information for the same biological features and contain the same genes at the same loci but possibly each have different alleles (that is, different genetic information) at those genes.
o When a segment of DNA from one chromosome is inserted into another nonhomologous chromosome, the resulting mutation is called a translocation.
o In inversion the orientation of a section of DNA is rotated 180 degrees in the chromosome.
o Translocation and inversion can be caused by transposition, which takes place in BOTH prokaryotic and eukaryotic cells.
o The DNA segments of transposable elements or transposons can excise themselves from a chromosome and reinsert themselves at another location.
o They are usually flanked by identical nucleotide sequences (the circles in this case).
o When moving, the transposon may excise itself from the chromosome and move; it may copy itself and move, or copy itself and stay, moving the copy.
o Transposition is one mechanism that a somatic cell, of a multicellular organism, can alter its genetic makeup without meiosis.
o A mutation can be a forward mutation or a backward mutation.
o These terms refer to an already mutated organism that is mutated again.
o The mutations can be forward, tending to change the organism even more from its original state, or backward, tending to revert the organism back to its original state.
o The original state is called the wild type.
o Forward mutations usually occur more than reverse mutations.
"Mutations are usually bad"
Mutation Citations
- Germline mutation: de novo mutation in reproductive lineage cells. Genes Genet Syst . 2019 Apr 9;94(1):3-12.
- Evolution of recombination rates between sex chromosomes. Philos Trans R Soc Lond B Biol Sci . 2017 Dec 19;372(1736):20160456.
- Meiotic recombination mechanisms. C R Biol . Jul-Aug 2016;339(7-8):247-51.
- En passant mutagenesis: a two step markerless red recombination system. Methods Mol Biol . 2010;634:421-30.
- λ Recombination and Recombineering. EcoSal Plus . 2016 May;7(1).
- Meiotic Recombination: The Essence of Heredity. Cold Spring Harb Perspect Biol . 2015 Oct 28;7(12):a016618.
- Recombination, Pairing, and Synapsis of Homologs during Meiosis. Cold Spring Harb Perspect Biol . 2015 May 18;7(6):a016626.
Share
Similar Post:
