-
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:
-
Protein Synthesis: Definition, Steps, Site, and Diagram
Continue ReadingProtein Synthesis or Protein Translation
o Protein Translation is the process of protein synthesis directed by mRNA.
o mRNA is the template which carries the genetic code from the nucleus to the cytosol in the form of codons.
o The mRNA message is always written in the 5’ → 3’ direction, and the polypeptide chain is synthesized starting with its N-terminal residue.
o The mRNA is always read in groups of 3 nucleotides.
o Prokaryotic translation may occur simultaneously with transcription.
o tRNA contains a set of nucleotides that is complimentary to the codon called the anticodon.
o tRNA sequesters the amino acid that corresponds to its anticodon.
o The 5’ base of the anticodon is capable of a “wobble” in its position during translation, allowing it to make alternative hydrogen bonding arrangements with several different codon bases.
o Identity elements (not on the anticodon) as well as the anticodon determine which amino acid is bound to the tRNA by the aminoacyl-tRNA synthetases.
o rRNA with protein makes up the ribosome, which provides the site for translation to take place.
o The ribosome is composed of a small subunit and a large subunit made from rRNA and many separate proteins.
o The ribosome and its subunits are measured in terms of sedimentation coefficients given in Svedberg units (S).
o The sedimentation coefficient gives the speed of a particle in a centrifuge, and is proportional to mass, and related to shape and density.
Protein Synthesis Diagram
o Prokaryotic ribosomes are smaller than eukaryotic ribosomes.
o Prokaryotic ribosomes are made from a 30S and a 50S subunit and have a combined sedimentary coefficient of 70S.
o Eukaryotic ribosomes are made from a 40S and a 60S subunit and have a combined sedimentary coefficient of 80S.
o The complex structure of ribosomes requires a special organelle called the nucleolus in which to manufacture them.
o Prokaryotes don’t have a nucleolus, but synthesis of prokaryotic ribosomes is similar to that of eukaryotic ribosomes.
o Although the ribosome is assembled in the nucleolus, the small and large subunits are exported separately to the cytoplasm.
o rRNA is synthesized in the nucleolus.
o After posttranscriptional processing in a eukaryote, mRNA leaves the nucleus through the nuclear pores and enters the cytosol.
o With the help of initiation factors, the 5’ end attaches to the small subunit of a ribosome.
o A tRNA possessing the 5’-CAU-3’ anticodon sequesters the amino acid methionine and settles in the P site (peptidyl site).
o This is the signal for the large subunit to join and form the initiation complex.
o This process is called initiation. Now elongation of the polypeptide begins.
"Prokaryotic ribosomes are smaller than eukaryotic ribosomes"
o A tRNA with its corresponding amino acid attaches to the A site (aminoacyl site) at the expense of two GTPs.
o The C-terminal of the methionine attaches to the N-terminal of the incoming amino acid at the A site in a dehydration reaction catalyzed by peptidyl transferase, an activity possessed by the ribosome.
o In an elongation step known as translocation, the ribosome shifts 3 nucleotides along the mRNA toward the 3’ end.
o The tRNA that carried methionine is carried to the E site where it can exit the ribosome.
o Then the tRNA that is in the A site is moved to the P site.
o Translocation requires the expenditure of another GTP.
E ⇒ EXIT
P ⇒ PROCESS
A ⇒ ARRIVE
Protein Synthesis Steps
o Translation ends when a stop codon (UAA, UAG, UGA) is reached in a step called termination. When a stop (or nonsense) codon reaches the A site, proteins known as release factors bind to the A site allowing a water molecule to add to the end of the polypeptide chain.
o The protein is released and the rRNA breaks up into its subunits and waits for it to be used again in another round of translation Even as the polypeptide is being translated, it begins folding.
o The amino acid sequence determines the folding conformation and the folding process is assisted by proteins called chaperones.
o In post-translational modification, sugars, lipids, or phosphate groups may be added to the amino acids.
o The polypeptide may be cleaved in one or more places, or other polypeptides might join to form a quaternary structure.
o Translation may take place on a free floating ribosome in the cytosol producing proteins that function in the cytosol or a ribosome may attach itself to the rough ER during translation and inject the protein into the ER lumen.
o Proteins injected into the ER lumen are destined to become membrane bound proteins of the nuclear envelope, ER, Golgi, plasma membrane, or to be secreted from the cell.
o Free floating ribosomes are identical in structure to the ribosomes that attach to the ER.
o The growing polypeptide may or may not cause the ribosome to attach to the ER depending upon the polypeptide.
o A 20 amino acid sequence called a signal peptide near the front of the peptide is recognized by protein-RNA signal- recognition particles (SRPs) that carries the entire ribosome complex to a receptor on the ER.
o The peptide is actually pulled through the membrane through an ATP driven process.
o The signal peptide is also usually removed by an enzyme.
"Signal peptides may also be attached to polypeptides to target them to mitochondria, the nucleus, or other organelles"
Protein Synthesis Citations
- Protein synthesis. Annu Rev Biochem . 1966;35:723-58.
- A Quick Guide to Small-Molecule Inhibitors of Eukaryotic Protein Synthesis. Biochemistry (Mosc) . 2020 Nov;85(11):1389-1421.
- Protein synthesis. Annu Rev Biochem . 1973;42:397-438.
- Bacterial Protein Synthesis as a Target for Antibiotic Inhibition. Cold Spring Harb Perspect Med . 2016 Sep 1;6(9):a025361.
- Protein synthesis and quality control in aging. Aging (Albany NY) . 2018 Dec 18;10(12):4269-4288.
- Protein Synthesis Initiation in Eukaryotic Cells. Cold Spring Harb Perspect Biol . 2018 Dec 3;10(12):a033092.
Share
Similar Post:
-
Genetic Code: Definition, Characteristics, Table, and Facts
Continue ReadingGenetic Code Chart
o There are four different nucleotides in RNA that together must form an unambiguous code for the 20 common amino acids.
o The code must be a combination of any three nucleotides.
o However, any three nucleotides gives 4^3 = 64 possible combinations.
o These are more possibilities than there are amino acids.
o Thus more than one series of three nucleotides may code for any amino acid; The code is degenerative.
o The code is also unambiguous (clear), one codon codes for only one amino acid and never two.
o In addition the code is almost universal (mitochondria present an exception), nearly every living organism uses the same code.
Genetic Code Chart
o Redundancy is usually expressed in the third letter of the codon (3’ position).
o Three consecutive nucleotides on a strand of mRNA represent a codon.
o All but three possible codons code for amino acids, so there are only 61 codes for amino acids.
o The remaining codons UAA, UGA, and UAG are stop codons, which signal an end to the protein synthesis.
o AUG is a start codon and it also codes for amino acid methionine.
o Codon changes have been opposed by the most intense selective pressure during evolution.
o By convention, a sequence of RNA nucleotides is written 5’ → 3’ We must understand probabilities.
o For instance, you must be able to figure out how many possible codons exist.
o As discussed above, four possible nucleotides can be placed in 3 spots giving 4^3 = 64.
o What about this? “A polypeptide contains 100 amino acids. How many possible amino acid sequences are there for this polypeptide? ”
o Well first we know that there are 20 amino acids and from the information we just received we know that there are 100 spots to place them. So it must be 20^100 possible sequences.
Genetic Code Chart Citations
Share
Similar Post:
-
DNA Technology: Definition, Types, and Facts
Continue ReadingDNA Technology: Nucleic Acid Hybridization
o When heated or immersed in high concentration salt solution or high pH solution, the hydrogen bonds connecting the two strands in a double stranded DNA molecule are disrupted, and the strands separate; the DNA molecule is said to be denatured or melted.
o Denatured DNA is less viscous (less syrupy), denser, and more able to absorb UV light.
o Separated strands will spontaneously associate with their original partner or any complementary nucleotide sequence.
o Thus, the following double stranded combinations can be formed through nucleic acid hybridization.
o DNA-DNA, DNA-RNA, RNA-RNA
Restriction Enzymes
o One method bacteria use to defend themselves from viruses is to cut the viral DNA into fragments with restriction enzymes.
o The bacteria protect their own DNA from these enzymes by methylation.
o Methylation is usually, but not always, associated with inactivated genes.
o Restriction enzymes (also called restriction endonucleases) digest/cut nucleic acid only at certain nucleotide sequences along the chain.
o Such a sequence is called a restriction sire or recognition sequence.
o Typically, a restriction site will be a palindromic sequence four to six nucleotides long.
o Most restriction endonucleases cleave the DNA strand unevenly, leaving complementary single stranded ends.
o These ends can reconnect through hybridization and are termed sticky ends.
o Sticky ends are produced by cutting the DNA in a staggered manner within the recognition site producing single-stranded DNA ends.
o It can also be cut so that it has blunt ends.
o Two DNA fragments cleaved by the SAME endonuclease can be joined together regardless of the origin of the DNA.
o Such DNA is called recombinant DNA; it has been artificially recombined.
Plasmid
o A plasmid is an extra-chromosomal DNA molecule separate from the chromosomal DNA which is capable of replicating independently of the chromosomal DNA.
o In many cases, it is circular and double-stranded.
o Plasmids usually occur naturally in bacteria, but are sometimes found in eukaryotic organisms.
o Plasmids used in genetic engineering are called vectors.
o Plasmids serve as important tools in genetics and biotechnology labs, where they are commonly used to multiply (make many copies of) or express particular genes.
o The gene to be replicated is inserted into copies of a plasmid containing genes that make cells resistant to particular antibiotics and a multiple cloning site, which is a short region containing several commonly used restriction sites allowing the easy insertion of DNA fragments at this location.
o Next, the plasmids are inserted into bacteria by a process called transformation.
o Then, the bacteria are exposed to the particular antibiotics.
o Only bacteria which take up copies of the plasmid survive , since the plasmid makes them resistant.
o Eukaryotic DNA contains introns.
o Since bacteria have no mechanism for removing introns, it is useful to clone DNA with no introns.
o In order to do this, the mRNA produced by the DNA is reverse transcribed using reverse transcriptase.
o The DNA product is called complementary DNA (cDNA).
o Adding DNA polymerase to cDNA produces a double strand of the desired DNA fragment (an entire DNA double helix free of introns).
Polymerase Chain Reaction (PCR)
o The polymerase chain reaction (PCR) is a technique to amplify a single or few copies of a piece of DNA across several orders of magnitude, generating millions or more copies of a particular DNA sequence.
o The method relies on thermal cycling, consisting of cycles of repeated heating and cooling of the reaction for DNA melting and enzymatic replication of the DNA.
o Primers (short DNA fragments) containing sequences complementary to the target region along with a DNA polymerase.
o Almost all PCR applications employ a heat-stable DNA polymerase, are key components to enable selective and repeated amplification.
o As PCR progresses, the DNA generated is itself used as a template for replication, setting in motion a chain reaction in which the DNA template is exponentially amplified.
o Starting with a single fragment, 20 cycles produces 2^20 copies
Southern Blotting
o Southern blotting is a technique used to identify target fragments of known DNA sequence in a large population of DNA.
o It is separated according to size by gel electrophoresis.
o The anode for Southern blotting is positive!.
o The cathode for Southern blotting is negative.
Northern Blotting
o Northern blotting is just like Southern blotting but it identifies RNA fragments.
Restriction Fragment Length Polymorphism (RFLP)
o Restriction fragment length polymorphism (RFLP) analysis identifies individuals as opposed to individual specific genes.
o The DNA of different individuals possesses different restriction sites and varying distances between restrictions sites.
o After fragmenting the DNA sample with endonucleases, southern blotting is used and produces bands distinct to the individual.
o The genome of one human differs from the next at about one nucleotide in every 1000.
o These differences have been called single nucleotide polymorphisms (SNPs)
DNA Technology Citations
Share
Similar Post:
-
Post Transcriptional Modification: Definition, Types, and Mechanism
Continue ReadingPost Transcriptional Modification in Prokaryotes
o rRNA and tRNA go through posttranscriptional processing.
o Almost all mRNA is directly translated to protein.
Post Transcriptional Modification in Eukaryotes
o Each type of RNA undergoes posttranscriptional processing.
o Posttranscriptional processing allows for additional gene regulation.
o The initial mRNA nucleotide sequence arrived at through transcription is called the primary transcript (also called pre-mRNA, or heterogeneous nuclear RNA [hnRNA]).
Type of Post Transcriptional Modification
The primary transcript is processed in three (3) ways:
1) addition of nucleotides
2) deletion of nucleotides
3) modification of nitrogenous bases
o Even before the eukaryotic mRNA is completely transcribed, its 5’ end is capped in a process using GTP.
o The 5’ cap serves as an attachment site in protein synthesis and as a protection against degradation by exonucleases.
o The 3’ end is polyadenylated with a poly A tail, also to protect it from exonucleases.
o The primary transcript is much longer than the mRNA that will be translated into a protein.
o This is due to noncoding regions existing in the primary transcript.
o These regions are called introns.
o Introns = noncoding regions of DNA in a gene; generally much longer than exons; Introns stay IN nucleus
o Exons = coding regions of DNA in a gene; Exons EXIT nucleus
o Enzyme-RNA complexes called small nuclear ribonucleoproteins (snRNPs “snurps”) recognize nucleotide sequences at the ends of the introns.
o Several snRNP’s associate with proteins to form a complex called a spliceosome.
o Inside the spliceosome, the introns are looped bringing the exons together.
o The introns are then excised by the spliceosome and the exons are spliced together.
o The exons of some genes may be spliced together in different order allowing them to encode for different polypeptides.
Citations
Share
Similar Post:
