AQA A level Biology Topic 4 - Genetic information, variation and relationships between organisms notes (A* achieved)

AQA AS Biology notes Topic 4 – Genetic information, variation and relationships between organisms
Topic 4 – Genetic information, variation and relationships
between organisms
Table of Content

4.1 DNA, genes & chromosomes.........................................................2
4.2 DNA & protein synthesis...............................................................2
4.3 Mutation......................................................................................6
Gene mutation.................................................................................................................................................................................6
Meiosis.............................................................................................................................................................................................7


4.4 Genetic diversity and adaptation................................................10
4.5 Species & taxonomy...................................................................12
4.6 Biodiversity within a community.................................................14
4.7 Investigating diversity.................................................................16




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, AQA AS Biology notes Topic 4 – Genetic information, variation and relationships between organisms

4.1 DNA, genes & chromosomes
DNA stored in eukaryotes vs prokaryotes
Eukaryotes Prokaryotes
 Long  Short
 Linear  Circular
 Associated with histones  Not associated with histones

Genes
 A base sequence of DNA that codes for
o Amino acid sequence of polypeptide
o A functional RNA eg ribosomal RNA & tRNAs
 A gene occupies a fixed position, called locus, on a particular DNA molecules

The nature of genetic code
Describe the features of a gene which enable it to code for a particular protein (4)
 Triplet code: a sequence of 3 bases coded for 1 specific amino acid
o Eg UAU codes for tyrosine
 Universal: same triplet codes for the same amino acid in all organisms
o Eg UAU codes for tyrosine in all organisms
 Non-overlapping: tRNA reads 3 bases, then the next 3
o Eg 123456 is read 123 456, not 123 234 345 456
 Degenerate: some amino acids are coded for more than 1 codon
o Eg tyrosine can be coded for by UAU or UAC


In eukaryotes, much of DNA doesn't code for polypeptides
 Between genes, non-coding multiple repeats
 Within genes, only exons code for amino acid sequences, which are separated by one or more non-
coding sequences, called introns


4.2 DNA & protein synthesis
Structure of messenger RNA (mRNA)
 Single, straight chain of
mononucleotides
 Has codons (3 bases on mRNA that code
for an amino acid)




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, AQA AS Biology notes Topic 4 – Genetic information, variation and relationships between organisms


Structure of transfer RNA (tRNA)
 Single stranded RNA folded over into a ‘cloverleaf’
shape held by hydrogen bonds between bases
 Has an amino acid attachment site on the top
 Has anticodon (3 specific bases) at the bottom,
which binds to complementary codons on mRNA




Similarities / differences between structure of mRNA & tRNA molecules
Similarities Difference
 Both single polynucleotide mRNA tRNA
strand
 Doesn't have H bonds / base  Have H bonds / base
pairing pairing
 Linear / straight chain  Cloverleaf
 Doesn't have amino acid  Have amino acid
attachment site attachment site
 Has codons  Has anticodon
 Different mRNA have different  All have same length
length
 Has more nucleotides

Function of RNA - transfer genetic info from DNA to ribosomes to make proteins

Protein synthesis




Transcription - production of mRNA from DNA in nucleus (DNA → pre-mRNA)




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