L8_2025_-_DNA

Transformation

A change in genotype and phenotype due to assimilation of external DNA by a cell.

Semi-conservative replication

the two strands of the parental molecule separate, each functions as a template for synthesis of a new complementary strand. consists of one parental and one newly synthesised strand

DNA polymerase

An enzyme that synthesizes new strands of DNA by adding nucleotide units to a growing chain.

X-ray diffraction

A technique used by Rosalind Franklin to determine the helical structure of DNA.

Anti-parallel strands

Refers to the orientation of the two strands of DNA running in opposite directions.

Okazaki fragments

Short sequences of DNA synthesized on the lagging strand during DNA replication.

Chargaff's rules

States that in DNA, the amounts of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C).

Meselson-Stahl experiment

An experiment that provided evidence for the semi-conservative mechanism of DNA replication.

Hershey-Chase experiment

An experiment that confirmed DNA as the genetic material by showing only DNA enters bacterial cells during infection.

Replicative fork

Y-shaped region where the DNA strands are being unwound and replicated.

Helicase

An enzyme that unwinds the DNA double helix at the replication forks.

Topoisomerase

An enzyme that prevents DNA tangling ahead of the replication fork.

Primase

An enzyme that synthesizes RNA primers needed for DNA replication initiation.

DNA composition

A polymer of nucleotides, consisting of a sugar (deoxyribose), a phosphate, and nitrogenous bases (A, T, G, C).

purine

nucleotides consisting of 2 fused rings - A and G

pyrimidines

nucleotides consisting of a single ring - C, T, and U.

polarity and structure of base pairs

Bases are non polar, hydrophobic on the inside and the polar hydrophilic phosphate is on the outside

Conservative model

the two parental strands re-associate after acting as templates for new strands, restoring the parental double helix, original double helix is intact and unchanged

Dispersive Model

each strand of both daughter molecules contains a mixture of old and newly synthesised DNA, parental DNA is mot kept intact but dispersed into both strands

Initiation of Replication

proteins recognise the DNA sequence and bind to it, opening up the double helix, in the replication bubble, there are Y-shaped replication forks where the new strands are being elongated, replication starts at multiple sites where the parental strands separate to form replication bubbles, the bubbles expand laterally as DNA replication proceeds in both directions, the replication bubbles fuse and synthesis of the daughter strands are complete

Directionality of DNA Replication

Due to anti-parallel strands and enzymes only being able to extend chains in only one direction (5’→3’), the two strands cannot both be replicated continuously, resulting in a leading strand (continuous replication) and a lagging strand (discontinued replication)