DNA Replication: The Semi-Conservative Model

These flashcards cover the key molecular mechanisms, enzymes, and clinical implications of DNA replication as described in the provided notes.

DNA replication

The process by which a cell copies its genetic material.

Semi-conservative model

A mechanism of DNA replication where a new double-stranded DNA molecule contains one original (parental) strand and one newly synthesized strand.

Anti-parallel arrangement

The structure of DNA where the two strands run in opposite directions, one from $5'$ to $3'$ and the other from $3'$ to $5'$, based on their chemical ends.

$5'$ end

The end of a DNA strand that has a free phosphate group.

$3'$ end

The end of a DNA strand that has a free hydroxyl ($-OH$) group.

Synthesis phase ($S$ phase)

The specific time in the cell cycle during which DNA replication occurs before the cell undergoes mitosis ($M$ phase).

Origins of replication

Specific locations on the chromosome where DNA replication begins.

Helicase

An enzyme that unwinds the DNA double helix by breaking the weak hydrogen bonds between complementary base pairs ($Adenine-Thymine$ and $Guanine-Cytosine$).

Replication fork

A Y-shaped region where the DNA double helix is unwound and active DNA synthesis occurs.

Single-Strand Binding ($SSB$) proteins

Proteins that attach to separated DNA strands to prevent them from re-joining and protect them from enzymatic degradation.

Topoisomerase

An enzyme that relieves physical tension (supercoiling) ahead of the replication fork by making temporary cuts in the DNA backbone and then resealing them.

DNA Polymerase

The primary enzyme that builds the new DNA strand by adding complementary nucleotides to an existing chain in the $5'$ to $3'$ direction.

Primase

An enzyme that creates a short sequence of RNA called a primer to provide a starting point for DNA Polymerase.

RNA primer

A short RNA sequence that provides the necessary free $3'$-hydroxyl ($-OH$) group for DNA Polymerase to begin synthesis.

Leading Strand

The DNA strand synthesized continuously in the same direction that the replication fork is opening.

Lagging Strand

The DNA strand synthesized discontinuously in the opposite direction of the replication fork's movement.

Okazaki fragments

Short segments of DNA produced during the discontinuous synthesis of the lagging strand.

DNA Ligase

An enzyme that joins disconnected Okazaki fragments together by forming covalent bonds, creating one continuous DNA strand.

Proofreading

The ability of DNA Polymerase to detect and correct mistakes by using a special $3'$ to $5'$ exonuclease activity to remove incorrect nucleotides.

Acyclovir

An antiviral drug that lacks a true $3'$-hydroxyl ($-OH$) group, leading to chain termination when incorporated into viral DNA.

Telomeres

Long stretches of repetitive, non-coding DNA ($TTAGGG$ in humans) that cap the ends of linear chromosomes to protect vital genes.

Telomerase

An enzyme that acts as a reverse transcriptase to add missing DNA sequences back to the telomeres, often active in stem cells and cancer cells.

Senescence

A state where a cell permanently stops dividing or undergoes programmed cell death due to its telomeres becoming critically short.