dna structure and replication

Question 1

what is dna

Answer 1

double helix comprising of 2 strands of nucleotides, each nucleotide formed of a sugar, phosphate group, and 1 of 4 nitrogenous bases

Question 2

what is a gene

Answer 2

unit of inheritance, made up of a sequence of nucleotides that codes for a polypeptide

Question 3

what is a chromosome

Answer 3

thread-like structure made up of DNA and protein

Question 4

relationship between DNA and chromosomes

Answer 4

a DNA molecule coils around histone proteins to form chromatin, which further compacts to form chromosomes

Question 5

how are nucleotides formed

Answer 5

condensation reactions, with
covalent bonds between carbon-1 of sugar and nitrogenous base and carbon-5 of sugar and phosphate group,
resulting in the formation of 2 water molecules

Question 6

how are dinucleotides formed

Answer 6

condensation reaction,
covalent bond between 5’-phosphate group of one nucleotide and 3’OH group of another nucleotide (phosphodiester bond),
resulting in formation of a water molecule

nucleotides always added to 3’ end

Question 7

rule of complementary base pairing

Answer 7

adenine bonds with thymine (2 hydrogen bonds)
cytosine bonds with guanine (3 hydrogen bonds)

Question 8

what contributes to DNA stability

Answer 8

• hydrogen bonds between the complementary base pairs
• sugar phosphate backbone formed by alternating sugar and phosphate molecules

Question 9

why is DNA replication semi-conservative

Answer 9

parent strand is used as a template for the synthesis of daughter strands

Question 10

enzymes involved in dna replication (in order)

Answer 10

helicase
primase
DNA polymerase III
DNA polymerase I
DNA ligase

Question 11

function of helicase

Answer 11

unwinds double helix by breaking hydrogen bonds between complementary base pairs in the parental strands

Question 12

function of primase

Answer 12

catalyses the synthesis of RNA primer to provide free 3’OH ends for DNA polymerase III

Question 13

function of DNA polymerase III

Answer 13

catalyses phosphodiester bond formation between incoming deoxyribonucleotides and the free 3’OH end of daughter strand

Question 14

function of DNA polymerase I

Answer 14

removes RNA primers and replaces them with deoxyribonucleotides

Question 15

function of DNA ligase

Answer 15

forms phosphodiester bonds between okazaki fragments to connect them

Question 16

process of DNA replication

Answer 16

1. begins at origin of replication
2. helicase unwinds double helix
3. each parental strand acts as template for synthesis of daughter strand
4. primase attaches to the unwound chain and catalyses synthesis of RNA primer
5. DNA polymerase III elongates daughter strand in the 5’ to 3’ direction
6. free deoxyribonucleotides are incorporated to the parental DNA strands by complementary base pairing
7. leading strand is synthesised continuously while lagging strand is synthesised discontinuously to form okazaki fragments
8. RNA primers are removed and replaced by deoxyribonucleotides by DNA polymerase I
9. nicks between okazaki fragments are filled by DNA ligase