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Biology OCR A level Genetics of living systems Summary Notes
Biology OCR A level Genetics of living systems Summary Notes
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1 of 6B19- Genetics of living systems
6.1.1 Cellular control
(a) Types of gene mutations and their possible effects on protein production and function
To include substitution, insertion or deletion of one or more nucleotides
AND
the possible effects of these gene mutations
(i.e. beneficial, neutral or harmful).
Key words:
Mutation- a change in the genetic material which may affect the phenotype of the organism.
Substitutions- a mutation where one or more nucleotides are substituted for another in a DNA
strand.
Deletion- a mutation where one or more nucleotides are deleted and lost from the DNA strand.
Insertion- a mutation where one or more extra nucleotides are inserted into a DNA strand.
Mutagens- chemical or physical agent which causes mutations.
Point mutation- when one base pair is replaced by another, usually has a limited effect on the
gene it occurs in.
Codon- a three-base sequence of DNA or RNA that codes for an amino acid.
Intron- non-coding part of DNA.
Translocation- when a section of one chromosome breaks off and joins another non-homologous
chromosome.
• The substitution of a single nucleotide changes the codon in which it occurs . If the new
codon codes for a different amino acid this will lead to a change in the primary structure of
the protein. Degenerate nature of the genetic code may mean that the new codon still
codes for the same amino acid leading to no change in the protein synthesised.
• Insertion or deletion of a nucleotide/s leads to a frameshift mutation. The triplet code
means that sequences of bases are transcribed consecutively in non-overlapping groups
of three- reading frame of a sequence of bases.
• Addition or deletion of a nucleotide moves the reading frame of the sequence of bases
which will change every successive codon from the point of mutation.
Effects of different mutations
• No effect: there is no effect on the phenotype of an organism because normally functioning
proteins are still synthesised. Could be for the same amino acid, doesn't interfere with
structure or could be a non-coding point of DNA called introns.
• Damaging: the phenotype of an organism is affected in a negative way because proteins
are no longer synthesised or they are non-functional, which can interfere with essential
processes.
• Beneficial: rare e.g. a mutation in a protein present in the cell surface membranes of
human cells means that HIV can’t bind and enter these cells, these people are immune to
infection from HIV.
Causes of mutations
• Rate of mutations increase by mutagens.
• Loss of a purine base (depurination) or a pyrimidine base (depyrimidination) often occurs
spontaneously. The absence of a base can lead to the insertion of an incorrect base
through complementary base pairing during DNA replication.
• Free radicals, oxidising agents, can affect the structures of nucleotides and also disrupt
base pairing during DNA replication.
Beneficial mutations
• Ability to digest lactose is a recent mutation as the majority of mammals become lactose
intolerant after they cease to suckle. Makes milk an effective part of the diet, increasing
the amount of calcium in the diet and reducing the presence of osteoporosis.
• Found mainly in European population as they are more likely to farm cattle.
• Natural selection
Chromosome mutations
, 2 of 6
• Gene mutations occur in single genes or sections of DNA whereas chromosome mutations
affect the whole chromosome or number of chromosomes within a cell.
• Can also be caused by mutagens and normally occur during meiosis.
Changes in chromosome structure normally include:
• Deletion: section of chromosome breaks off and is lost within the cell.
• Duplication: sections get duplicated on a chromosome.
• Translocation: section of one chromosome breaks off and joins another non-homologous
chromosome.
• Inversion: section of chromosome breaks off, is reversed, and then joins back onto the
chromosome.
Three types of point mutation:
- Silent
- Missense
- Nonsense
Silent mutations
• Point mutation involving a change to the base triplet, where that triplet still codes for the same
amino acid.
• Primary structure of the protein and therefore the secondary and tertiary structure, is not altered.
Missense mutations
• Change to the base triplet sequence that leads to a change in the amino acid sequence in a
protein.
• Within a gene, such a point mutation may have a significant effect on the protein produced. The
alteration to the primary structure leads to a change to the tertiary structure of the protein,
altering its shape and preventing it from carrying out its usual function.
• E.g. Sickle cell anaemia, causes deoxygenated haemoglobin to crystallise within erythrocytes,
causing them to become sickle shaped, blocking capillaries and depriving tissues of oxygen.
Nonsense mutations
• May alter a base triplet, so that it becomes a termination (stop) triplet. This particularly
disruptive point mutation results in truncated protein that will not function.
• This abnormal protein will most likely be degraded within the cell.
• E.g. Duchenne muscular dystrophy.
(b) The regulatory mechanisms that control gene expression at the transcriptional level, post-
transcriptional level and post-translational level
To include control at the,
• transcriptional level: lac operon, and transcription factors in eukaryotes.
• post-transcriptional level: the editing of primary mRNA and the removal of introns to produce
mature mRNA.
• post-translational level: the activation of proteins by cyclic AMP.
Key words:
Housekeeping genes- enzymes which are necessary for reactions present in metabolic
pathways are constantly required, and the genes that code for.
Heterochromatin- tightly packed DNA.
Euchromatin- loosely packed DNA.
Acetylation- addition of acetyl group.
Methylation- addition of methyl group.
Epigenetics- external control of genetic regulation.
Structural genes- genes that code for structural proteins or enzymes not involved in DNA
regulation.
Regulatory gene- a gene that codes for proteins involved with DNA regulation.
Repressor protein- protein that binds to operator affecting the rate of transcription.
Exons- separate the coding of expressed regions.
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