7.1 DNA structure and replication

Question 1

DNA structure

Answer 1

composition: double stranded molecule
orientation: nitrogenous bases are closely packed together on the inside and phosphastes form outer backbone
shape: molecule twins at regular intervals to form a helix
- equal number of purines (A+G) and pyrimidines (C+T)
- strands run antiparallel
- adenine and thymine paired via 2 H bonds whereas guanine and cytosine paired via 3 H bonds

Question 2

mechanisms for DNA replication

Answer 2

• complementary base pairing
• bi-directional

Question 3

DNA replication

Answer 3

helicase:
- unwinds and separates double-stranded DNA by breaking H bonds between base pairs
- occurs at specific regions, creating a replication fork of 2 strands running in antiparallel directions
DNA gyrase:
- refuses strain created by unwinding DNA by helicase
- relaxes supercoils
single stranded binding proteins:
- bind to DNA strands after they have been separated and prevent strands from re-annealing
- also help to prevent single stranded DNA from being digested by nucleases
DNA primase:
- generates short RNA primer on each template strand
- provides initiation point for DNA pol III, which can extend nucleotide chains but not start one
DNA pol III:
- attaches to 3’ end of primer and covalently joins free nucleotides together in 5’ to 3’ direction
- moves in opposite directions on the strands (leading strand = moving towards replication fork and can synthesize continuously, lagging strand = moving away from replication fork and synthesizes in pieces)
DNA pol I:
- removes RNA primers from lagging strand and replaces them with DNA nucleotides
DNA ligase:
- joins Okazaki fragments together to form continuous strand

Question 4

tandem repeats

Answer 4

long stretches of DNA made up of repeating elements. can identify and compare individuals via their DNA profiles.

Question 5

nucleosomes

Answer 5

• help supercoil DNA, resulting in a greatly compacted structure that allows for more efficient storage
• consists of a molecule of DNA wrapped around 8 histone proteins

Question 6

organization of eukaryotic DNA

Answer 6

• complexed with 8 histone proteins to form a complex called a nucleosome
• nucleosomes are linked by an additional histone protein to form s strong of chromatosomes
• then coil to form solenoid structure (6 chromatosomes per turn) which is condensed to form fibre
• fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin
• chromatin will supercoil during cell division to form chromosomes that are visible