Chromosomes, Genes, DNA
○ A gene is a series of DNA nucleotides that generally codes for the production of a single polypeptide or mRNA, tRNA, rRNA.
○ The entire DNA sequence of an organism is called the genome.
○ Eukaryotes have more than one copy of some genes, while prokaryotes have only one copy of each gene.
○ Genes are often referred to as unique sequence DNA; while regions of non-coding DNA found only in eukaryotes are called repetitive sequence DNA.
○Generally speaking: One gene; one polypeptide. One exception is posttranscriptional processing of RNA.
Chromatin Structure
○ Negatively charged DNA loops twice around.
○Histone octamer (2 each of the positively charged H2A, H2B, H3, and H4) to form nucleosome bead.
"Histone octamer (2 each of the positively charged H2A, H2B, H3, and H4) to form nucleosome bead"
○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.
○ Repetitive DNA is found mainly in heterochromatin.
○The Central Dogma of gene expression is that DNA is transcribed to RNA, which is translated to amino acids forming a protein.
DNA
○ Four nitrogenous bases exist in DNA:
○ Purines: Two ringed structure
1) Adenine (less C=O)
2) Guanine (more C=O)
○ Pyrimidines: Single ring structure
3) Cytosine (less C=O)
4) Thymine (more C=O)
○ 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.
○ 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’
○In DNA, two strands are joined by the hydrogen bonds to make the structure called the double helix.
○ This model was proposed by Watson and Crick.
○ 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.
○ 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
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