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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)
2
, 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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