DNA

Question 1

Structure of DNA related to its functions

Answer 1

Sugar-phosphate backbone so provides strength
Double helix so backbone protects bases
Long molecule so stores lots of information
Helix so compact
Base sequence allows information to be stored
Double stranded so semi conservative replication can occur
Complementary base pairing so replication accurate
Hydrogen bonds easily broken for replication
Many hydrogen bonds so stable

Question 2

DNA replication

Answer 2

Two strands therefore semi-conservative replication possible
DNA helicase breaks hydrogen bonds between bases
Strands separate and both act as templates
Free DNA nucleotides diffuse into position
According to complementary base pairing A with T and C with G)
Hydrogen bonds hold nucleotides in place
DNA polymerase joins nucleotide together
To form phosphodiester bonds
DNA one parent and one new strand

Question 3

Transcription

Answer 3

DNA helicase
Breaks hydrogen bonds between bases
Strands separate
One DNA strand acts as a template
Free RNA nucleotides diffuse into position
According to complementary base pairing
A with U and C with G
RNA polymerase joins RNA nucleotides together
To form phosphodiester bonds
Pre-mRNA spliced to remove introns.
Mature mRNA contains exons

Question 4

Translation

Answer 4

mRNA leaves nucleus through nuclear pore
To ribosome
tRNA molecules bring amino acids to ribosome
Specific tRNA molecule for specific amino acid
Anticodon of tRNA is complementary to codon on mRNA
Two amino acids held close together on ribosome
Peptide bonds form between amino acids
tRNA detaches and collects another amino acid from cytoplasm
Ribosome moves along mRNA

Question 5

Mitosis

Answer 5

DNA replicated
Chromosomes condense or DNA wrapped round histones
Each chromosome has 2 identical chromatids due to replication
Chromosomes move to equator of cell
Centromere attach to individual spindle fibres
Spindle fibres contract
Chromatids separate and move to opposite poles
Nuclear envelope forms around each group of chromosomes
Cytokinesis

Question 6

Meiosis

Answer 6

DNA replicates
Chromosomes condense
Chromosomes associate in homologous pairs or bivalents
Crossing-over takes place
Centromeres join to spindle at equator of cell
Homologous chromosomes move to opposite poles
Cytokinesis
Chromosomes move to equator
Centromeres attach to spindles
Chromatids move to opposite poles
Cytokinesis

Question 7

Sources of variation

Answer 7

Mutation
Caused by mutagenic agent
Change in base sequence of DNA Deletion/ substitution
New protein coded for

Crossing over
Different combination of alleles

Independent assortment of homologous chromosomes/chromatids in meiosis I and II Different combinations of maternal and paternal chromosomes

Random fusion of gametes at fertilisation
New combinations of alleles