Unit 4 - Genetic information, variation and relationships between organisms
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Summary AQA Biology Unit 4: Genetic information, variation, and relationships between organisms full notes
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Unit 4 - Genetic information, variation and relationships between organisms
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AQA
Book
AQA Biology A Level Student Book
full notes hitting all areas of specification for unit 4 of AQA A Level biology
consisting from DNA and protein synthesis to biodiversity
with diagrams and definitions
highlighted key information
Unit 4 - Genetic information, variation and relationships between organisms
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3.4 genetic information, variation and relationships
between organisms
8 DNA, genes and protein synthesis
Source of DNA length Circular or linear Associated with
histone proteins?
Prokaryotic cell short circular no
Eukaryotic nucleus long linear yes
chloroplast/ short circular no
mitochondrion
A gene is a section of DNA located at a fixed point on a chromosome called its locus
A gene carries the coded genetic information that determines:
- The amino acid sequence
- A functional RNA molecule
Not all of the genome codes for proteins,
non coding = intron
Coding = exon
Some non coding sequences signal the end of transcription
Features of the genetic code:
● Non-overlapping = each triplet only reads once and triplets don't share any bases
● Degenerate = more than one triplet codes for the same amino acid, this reduces the
number of mutations that can be caused by base deletion, insertion or substitution
● Universal = almost every organism uses the same code
Genome = complete set of genes in a cell
Proteome = full range of proteins a cell can produce
mRNA Longer chain than Single stranded helix Chemically unstable; only
tRNA molecule present when required for
protein synthesis
tRNA Short chain Cloverleaf shaped; has Chemically stable
amino acid attachment
site on stalk and
anticodon mRNA binding
site on lower lobe
1
, Protein synthesis
- DNA template is
transcribed into a
messenger RNA molecule
in the nucleus
- mRNA base sequence is
then translated into an
amino acid sequence in
association with transfer
RNA on ribosomes in the
cytoplasm
-
Transcription
1. DNA helicase breaks hydrogen bonds between bases, unwinding DNA and causing
bases to be exposed
2. RNA polymerase moves along the DNA template strand (antisense strand) and
attaches free nucleotides to their complementary bases on the DNA via
phosphodiester bonds
3. RNA polymerase makes short strands of pre-mRNA until it comes to a terminator
sequence (stop codon)
4. (eukaryote only) Sections of pre-mRNA are spliced together to remove intron
sequences and join exon sequences to form mRNA
The template strand can be called the antisense strand
Translation
1. mRNA moves from the nucleus through nuclear pore to the cytoplasm and the start
codon attaches to a ribosome
2. A tRNA with a complementary anticodon moves to the ribosome and pairs with the
first mRNA codon
3. The ribosome moves along the mRNA to the next codon and again pairs up with
complementary tRNA, to bring the two amino acid carrying tRNAs together
4. Energy released from ATP is used to form a peptide bond between the amino acids
5. The ribosome moves to the third mRNA codon, releasing the first tRNA and pairing
up with a third
6. When the ribosome reaches a stop codon, the polypeptide is complete and the
mRNAs and tRNAs are released from the ribosome
7. The tRNA molecules released from the ribosome can then be reused
Up to 50 ribosomes can move along the same strand of mRNA behind one another so that
several proteins can be assembled simultaneously.
9 genetic diversity
Gene mutations can arise as a result of a mutation or during meiosis
Mutation = spontaneous change in base sequence of chromosomes
Base insertion and deletion is the addition or removal of a base. This is known as a
frameshift. This can change the primary and therefore tertiary structures of a protein.
2
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