Tag: Chromosomes
-
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.Â
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:
-
Chromosomes, Genes, DNA: Definition, Structure, and Functions
Continue ReadingChromosomes, Genes, DNA
â—‹ A gene is a series of DNA nucleotides that generally codes for the production of a single polypeptide or mRNA, tRNA, rRNA.
â—‹ The entire DNA sequence of an organism is called the genome.
â—‹ Eukaryotes have more than one copy of some genes, while prokaryotes have only one copy of each gene.
â—‹ Genes are often referred to as unique sequence DNA; while regions of non-coding DNA found only in eukaryotes are called repetitive sequence DNA.
â—‹Generally speaking: One gene; one polypeptide. One exception is posttranscriptional processing of RNA.
Chromatin Structure
â—‹ Negatively charged DNA loops twice around.
â—‹Histone octamer (2 each of the positively charged H2A, H2B, H3, and H4) to form nucleosome bead.Â
"Histone octamer (2 each of the positively charged H2A, H2B, H3, and H4) to form nucleosome bead"
â—‹Eukaryotic genes that are actively being transcribed by a cell are associated with regions of DNA called euchromatin, while genes not being actively transcribed are associated with tightly packed regions of DNA called heterochromatin.
â—‹ Repetitive DNA is found mainly in heterochromatin.
â—‹The Central Dogma of gene expression is that DNA is transcribed to RNA, which is translated to amino acids forming a protein.
DNA
â—‹ Four nitrogenous bases exist in DNA:
â—‹ Purines: Two ringed structure
1) Adenine (less C=O)
2) Guanine (more C=O)
â—‹ Pyrimidines: Single ring structure
3) Cytosine (less C=O)
4) Thymine (more C=O)
â—‹ Deamination of cytosine forms uracil
○ In nucleic acids, nucleotides are joined together by phosphodiester bonds between the third carbon of one dexoyribose and the phosphate backbone of a single strand of DNA with a 5 → 3 directionality.
â—‹ A phosphodiester bond is a group of strong covalent bonds between the phosphorus atom in a phosphate group and two other molecules over two ester bonds.
"In DNA, two strands are joined by the hydrogen bonds"
○ Nucleotides are written 5’ → 3’
â—‹In DNA, two strands are joined by the hydrogen bonds to make the structure called the double helix.
â—‹ This model was proposed by Watson and Crick.
â—‹ The members of each base pair can fit together within the double helix only if the two strands of the helix are antiparallel.
○ By convention the top DNA strand goes 5’ → 3’ and the bottom 3’ → 5’
○ Going in the 5’ → 3’ Direction is referred to as going downstream
○ Going in the 3’ → 5’ Direction is referred to as going upstream The two strands are complementary strands.
â—‹ The double helix contains two distinct grooves called the major groove and minor groove, which moves around once every 10 base-pairs.
Chromosomes, Genes, DNA Citations
Share
Similar Post:
