DNA

Question 1

DNA: a phosphate group + deoxyribose + an organic base

nucleic acids are made up of lots of nucleotides joined together by phosphodiester bonds

Answer 1

DNA is a double helix (2 polynucleotides wound round each other)

4 bases: Adenine = Thymine
               Guanine = Cytosine
∙DNA has a sugar phosphate back bone
∙bases are joined together by H+ bonds
∙strands are antiparallel: 5-3, 3-5

Question 2

RNA: a phosphate group + ribose + an organic base
∙single stranded
∙4 bases: Adenine = Uracil
                 Guanine = Cytosine
There are 3 types of RNA involved in protein synthesis
∙mRNA
∙tRNA
∙rRNA

Answer 2

mRNA: synthesised in the nucleus and carries the genetic code from the DNA to ribosomes in the cytoplasm

tRNA: molecules of tRNA transport specific amino acids to the ribosomes in protein synthesis

rRNA: found in cytoplasm, binding site for tRNA, site of translation

Question 3

Eukaryotes vs Prokaryotes:

E= DNA enclosed in nucleus
Diploid
chromosomes in pairs

P= no nucleus
DNA loose in cytoplasm
Haploid

Answer 3

∙the genetic code is a triplet code, 3 bases code for 1 amino acid

∙each section of DNA is called a gene

Question 4

Exons and introns:
∙in eukaryotes the pre-mRNA version of the code is much longer than the final mRNA and contains sequences of bases that need to be removed

∙introns are cut out using endonucleases, not translated

Answer 4

∙the sequence left are called exons, they are spliced together with ligases. this produces the final mRNA

Question 5

DNA replication:
∙semi conservative= each strand in the double helix acts as a template for the synthesis of a new complementary strand

this process takes us from one starting molecule to two daughter molecules

Answer 5

∙replication always starts at sites called ‘origins of replication’. specialised proteins recognise the origins, bind to this site and open up the DNA

∙as the DNA opens, 2 Y shaped replication forks form, together making up whats known as the ‘replication bubble’

Question 6

∙DNA helicase is the first replication enzyme to load on at the origin of replication. Helicase breaks hydrogen bonds between the bases

∙proteins called ‘single stranded binding proteins’ coat the seperated strands of DNA near the replication fork, keeping them from re-forming a double helix

Answer 6

Primers and primase:

∙DNA polymerases only add nucleotides to the 3’ end of the DNA

primase makes a RNA primer that provides a 3’ end so DNA polymerase can work on it

Question 7

Leading and lagging strands:

∙DNA polymerase can only make DNA in the 5-3 direction. As the strands are anti-parallel, both strands need to be made separately

∙5-3 towards the fork is made easily because DNA polymerase is moving in the same direction as the replication fork (LEADING STRAND)

∙5-3 away from the fork is made in fragments, because DNA polymerase is moving in the opposite direction to the fork, so it must come off and re-attach

OKAZAKI FRAGMENTS: fragments named after the Japanese scientist who discovered them

the lagging strand needs a new primer for each fragment

Answer 7

FINALLY:
RNA primers are removed and replaced by DNA

The nicks that remain after the primers, are replaced and get sealed by the enzyme DNA ligase