Chapter 4: DNA Structure & Transcription

bsc2010

Nucleoside

sugar (deoxyribose or ribose) + nitrogenous base

Nucleotide

nucleoside + phosphate group(s) (1, 2, or 3)

Purines (2 rings):

Adenine (A), Guanine (G)

Pyrimidines (1 ring):

Cytosine (C), Thymine (T), Uracil (U in RNA)

DNA base pairing

A ↔ T, C ↔ G

RNA base pairing

A ↔ U, C ↔ G

1′ carbon:

Base attaches

3′ carbon:

OH (hydroxyl) group attaches

5′ carbon:

Phosphate group attaches

phosphodiester bond

Phosphate group (5′ end) connects to the 3′ OH of the next nucleotide

Sugar-Phosphate Backbone

• Forms the structural framework of DNA and RNA.

• Nucleotides are linked by phosphodiester bonds between the 3′ OH and 5′ phosphate groups.

• Directionality: 5′ → 3′ (read and synthesized this way).

Two strands of nucleotides run

antiparallel (5′→3′ and 3′→5′).

A–T

(2 hydrogen bonds)

C–G

(3 hydrogen bonds)

Hydrogen bonds

between bases hold the strands together.

Central Dogma of Molecular Biology

DNA → RNA → Protein

Template DNA strand

3′ → 5′.

1. Initiation

• RNA polymerase binds to promoter region (includes TATA box in eukaryotes).

• Transcription factors help RNA polymerase bind.

• DNA unwinds and transcription begins.

2. Elongation

• RNA polymerase moves along the DNA template strand.

• Adds complementary ribonucleotides (A, U, C, G) to build mRNA.

• Phosphodiester bonds form between nucleotides.

• Energy comes from the cleavage of pyrophosphate from the incoming nucleotide.

3. Termination

• Termination sequence signals RNA polymerase to stop.

• RNA molecule detaches from DNA.

• In eukaryotes, pre-mRNA is formed (must be modified before use).

RNA polymerase II:

Enzyme that transcribes mRNA in eukaryotes.

Promoter:

DNA region where transcription begins.

TATA box:

A promoter sequence rich in thymine and adenine, helps position RNA polymerase.

Transcription factors:

Proteins that regulate RNA polymerase binding and activity.

Termination sequence:

DNA sequence signaling the end of transcription.

5′ Cap:

Added to the beginning; protects RNA and helps ribosome binding.

Splicing:

Removes introns (noncoding regions); joins exons (coding regions).

Polyadenylation:

Adds poly-A tail (about 100–250 adenines) to 3′ end to stabilize RNA and aid transport out of nucleus.

Alternative Splicing

• Different combinations of exons are joined to create multiple mRNA variants from a single gene.

• Increases protein diversity without increasing the number of genes.

Polyribosome (Polysome)

• A group of ribosomes simultaneously translating a single mRNA molecule.

• Allows multiple copies of a protein to be produced quickly from one mRNA transcript.