DNA structure and DNA replication

Question 1

nitrogenous bases

Answer 1

pyrimidines- thymine and cytosine
purines- adenine and guanine

Question 2

DNA polarity

Answer 2

DNA is antiparallel 5’ to 3’ going down and 3’ to 5’

Question 3

secondary structure

Answer 3

DNA is a double stranded: each molecule of DNA is composed of two polynucleotide chain that are joined together by formation of hydrogen bonds between bases
DNA strands are twisted to form a double helix
DNA is antiparallel

Question 4

factors stabilising DNA

Answer 4

hydrophobic effects- burying purine and pryrimidine rings in double helix interior
stacking interactions- stacked base pairs form van der waals contacts
hydrogen bonds- between base pairs

Question 5

nucleosomes

Answer 5

a nucleosome has a core of eight histone molecules around the DNA helix is wrapped
nucleosomes help to supercoil the DNA a greatly compared structure that allows for the more efficient storage
supercoiling helps to protect the DNA from damadge and also allows chromosomes to be mobile during mitoisis and meiosis

Question 6

chromatins and chromosomes

Answer 6

chromatin- nucleosomes are futher condensed into chromatin
chromosomes- chromatin fibres are organsised into loops and the loops into bands that form superstructure of chromosomes

Question 7

RNA

Answer 7

-single strand of nucletides
-ribose sugar
-A G C U

Question 8

DNA replication

Answer 8

hydrogen bonds break and helix opens by DNA helicase
each strand of DNA acts as a template for the synthesis of a new complementary strand
each strand of DNA acts as a template for the synthesis of a new complementary strand
repliaction produces two identivcal DNA double helices, each with one new and one old strand
semi conservative- each new strand formed will be identical to the original strand separated from the template
the two semi-conservative molecules formed will have an identical base sequence to the original molecule
DNA polymerase attaches free nucleotides

Question 9

DNA damage repair mechanisms

Answer 9

direct reversal: some DNA damaging chemical reactions can be directly undone by enzymes in the cell
excision repair: for damage to one or a few bases of DNA, base excersion repair andm nucleotide excision repair
double- stranded break repair: non homologous end joining and homologous recombination