Chromosomes, Genes, DNA: Definition, Structure, and Functions

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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. 

Gene Nucleotides: Definition, Functions, Types, and Examples

"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.


○ 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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