Summary OCR A-Level Biology 6.1.1 Genetics of Living Systems (Cellular Control)
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A Level Biology A for OCR Student Book
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Topics Included:
- Mutations and variation
- Contro...
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Genetics of Living Systems
19.1 Mutations and Variation
19.1 mutations and variation
- A mutation is a change in the sequence of bases in DNA.
- The change in sequence is caused by the substitution, deletion, or insertion 19.2 control of gene expression
of one of more nucleotides within a gene. 19.3 body plans
- They can result in silent, missense, nonsense, and frame shift mutations.
- They may result in a change to Change to Description Example of Genetic
the primary structure polypeptide DNA Disorder
produced by that gene. This may Substitution One nucleotide is exchanged for Sickle cell anaemia
then affect the folding of the another
protein and therefore its function. Insertion Extra nucleotides are added to the Huntington’s disease
- These mutations can have a few DNA sequence
different effects: Deletion Nucleotides are removed from the Cystic fibrosis
o Silent – no change to DNA sequence
amino acids primary
structure because more than one codon can code for the same amino acid. Or it may be in a
non-coding region.
o Missense – changes to amino acid sequence
o Nonsense – truncated protein (a codon is changed to a stop codon, stopping the protein short)
o Frame shift – triplets read differently due to insertion or deletion.
- Mutation is a random process and genes tend to mutate at their own rate. New mutations can create
new alleles for a gene.
- For a new mutation to be inherited it must occur in the gamete forming cells. If the gamete formed
from the cells with mutations in them are the ones that fuse during sexual reproduction all the cells of
that new organism will have a copy of that mutant allele.
- Mutations can be beneficial, neutral or harmful.
- Mutations in somatic cells will not be passed on but can contribute to ageing and cancer development.
- Mutagens increase the rate of mutation:
o X-rays, UV, and gamma radiation physically damage the DNA so it has to be repaired and
there may be mistakes.
o Chemicals such as nitrous acid results in changes of one DNA base for another, such as
cytosine converted to uracil so it bonds to adenine rather than guanine.
o Carcinogens such as benzene and tar chemicals
- Types of chromosome mutations:
o Insertion
o Deletion – a section of chromosome breaks off and is lost within the cell
o Inversion – a section of chromosome breaks off, is reversed, and then joins back onto the
chromosome
o Translocation – a section of one chromosome breaks off and joins another non-
homologous chromosome
o Non-disjunction – the chromosomes do not separate properly, leaving an abnormal number
of chromosomes
o Duplication – sections get duplication on a chromosome
19.2 Control of Gene Expression
- Everybody cell contains the entire genome.
- Some gene products are required by most cells all of the time these are called housekeeping genes.
- Other proteins are only needed by specific cells at specific time. The amount of the protein needs to be
regulated, or the activity of the proteins produced may need to be regulated in response to changes in
either the internal or external environment.
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