The deoxyribonucleotide (nucleotide).

1. Phosphate group
2. Deoxyribose sugar
3. Nitrogenous base (A, T, G, C)

5’ end has a free phosphate group; 3’ end has a free hydroxyl group on the sugar.

Adenine (A), Thymine (T), Guanine (G), Cytosine (C), Uracil (U)

Phosphodiester bonds connect sugar-phosphate backbone; hydrogen bonds hold complementary bases together.

5’ → 3’ direction along the new strand being synthesized.

1. DNA is helical.
2. DNA has two strands (double helix).
3. The strands are antiparallel and bases pair specifically (A-T, G-C).

Two strands run in opposite directions (one strand is 5’→3’ while the other is 3’←5’).

Each base pairs specifically with only one partner: A-T, G-C.

Alternating sugar (deoxyribose) and phosphate groups.

She produced X-ray diffraction images of DNA critical for determining the double helix.

1. Semiconservative: each daughter DNA has one old and one new strand.
2. Conservative: parent DNA remains intact; daughter DNA is completely new.
3. Dispersive: DNA strands are a mix of old and new segments.

Determine which alternative DNA replication hypothesis is supoort.

Grow E. coli in 15N (heavy) → switch to 14N (light) → centrifuge to separate DNA by density after replication cycles.

15N: heavy isotope containing 7 protons and 8 neutrons
14N: most common isotope of nitrogen containing 7 protons and 7 neutrons.

High: 15N DNA
Low: 14N DNA
Intermediate: hybrid 15N/14N DNA.

Semiconservative: 1st gen all hybrid (N14/N15 mix). 2nd gen 50% hybrid (N14/N15 mix) with 50% light (N14 only).
Conservative: 1st gen all heavy (N15 only), 2nd gen heavy (N15 only) + light (N14 only) .
Dispersive: 1st gen all hybrid (N14/N15 mix), 2nd gen all hybrid (N14/N15 mix).

1st generation: all hybrid → supports semiconservative replication and dispersive replication, rejects conservative replication
2nd generation: half hybrid, half light → confirms semiconservative.

Synthesizes new DNA strands by adding nucleotides complementary to the template strand.

Region where DNA strands have separated to allow replication.

DNA replication proceeds in both directions from the origin.

Bacteria: single origin of replication
Eukaryotes: multiple origins of replication per chromosome.

Primase lays RNA primer → DNA polymerase III extends → continuous synthesis toward replication fork.

Unwinds the double helix at the replication fork.

Stabilize single-stranded DNA and prevent re-annealing.

Hydrogen bonds between complementary bases between double strands.

Supercoiling.

Relieves supercoiling by cutting and rejoining DNA strands.

Synthesizes short RNA primers to start DNA synthesis.

Complex of enzymes performing replication; two replisomes per replication bubble (one per fork).

Leading: continuous synthesis toward fork (more efficient).
Lagging: discontinuous, away from fork.

1. Primase lays RNA primer.
2. DNA polymerase III extends primer.
3. DNA polymerase I removes RNA primers.
4. DNA polymerase fills in gaps.
5. DNA ligase seals Okazaki fragments.

Short DNA fragment synthesized on lagging strand.

Creates phosphodiester bonds between Okazaki fragments to create continuous strand.

Repeating DNA sequences at chromosome ends that protect coding DNA.

DNA Polymerase simply zips off completing the telomere.

Telomerase extend ends (telomeres) of chromosomes to prevent shortening.

RNA primer at very end cannot be replaced with DNA which can lead to chromosome shortening.

DNA would be shorter unless telomerase extends the end.

No; they are repetitive sequences that do not code for mRNA (TTAGGG in humans).

Extends telomeres to prevent chromosome shortening.

Very high: ~1 mistake per 10⁹–10¹⁰ nucleotides due to proofreading.

Proofreading by DNA polymerase and mismatch repair enzymes.

Recognize and fix errors in DNA after replication.