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chapter 19 genetics of living systems
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chapter 19 genetics of living systems (biology OCR A notes)
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19.1 mutations and variation2/10/23
Mutation = change in DNA base sequence caused by mutagens (e.g. ionising radiation)
Types of Point mutation
= affects only one base / nucleotide
- substitution mutation
- Insertion
- Deletion
insertion + deletion - called frameshift mutation - this is because the triplet code reading
is disrupted
- genetic code is degenerate multiple codons can code for a particular amino
acid → means a point mutation may have no effect → allowing us to make same
protein for normal functioning
Effects of mutation
- Neutral / silent - no effect ( due to degenerate nature of codon)
- Beneficial - enhanced function of protein
- Damaging - malfunctioning protein made
Other types of mutations
- Nonsense : codon → turns into stop codon → these usually have negative or
harmful effects on phenotypes
- Missense : e.g. amino acid ‘’A’’ turned into amino acid ‘B’( degenerate coding
has failed) ( this could be silent / beneficial /damaging)
- Chromosomal mutations : bigger sections of chromosomes have been
changed rather than 1 or 2 bases
Chromosome mutations:
= bigger sections of chromosomes have been changed rather than 1 or 2 bases
- Deletion : deleting part of chromosome
- Duplication : have part of DNA doubled
- Inversion : section of chromosome turned upside down
- Translocation: a section moved itself from one chromosome to another
19.2 control of gene expression
1) Control of gene expression - transcriptional
- Alter conditions to allow RNA polymerase to bind to the gene we want to
be transcribed
- E.g. prokaryotes - lac operon
- E.g. Eukaryotes - histone modification for chromatin remodelling
( DNA is wrapped around protein called histone sp that it becomes
, compact → chromatin - This is called heterochromatin which is tightly
wound DNA in cell division)
= heterochromatin
- Gene is inaccessible for RNA polymerase to reach in heterochromatin
- Transcription does not take place
- For transcription to happen , heterochromatin needs to be converted to a
another form aka Euchromatin ( loosely wound DNA in interphase)
= euchromatin
How does heterochromatin turn into → euchromatin?
- DNA is negatively charged due to negative phosphate groups , whilst
histones are positively charged . This is why they can bond together.
- ( adding acyl group to histones) / OR phosphorylation( add phosphate
group)
- Adding acyl/phosphate decreases the positive charge of histones → so
they won't be as tightly bound to histones so its more loose
How does turn euchromatin into →heterochromatin euchromatin?
- Methylation (adding methyl group which increases hydrophobic nature of
histones → more inviting to wrap around histones → heterochromatin)
Example
the Lac operon in prokaryotes
Operon = a group of genes controlled by same regulatory mechanism +
expressed at the same time
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