Summary OCR A Level Biology A* Cellular Control Detailed Notes
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Course
Module 6 - Genetics, evolution and ecosystems
Institution
OCR
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6.1 Cellular Control Notes
6.1.1 Gene Muta2ons
- Gene muta)on = change in sequence of base pairs in DNA molecule -> may result in changed polypep)de
- Occur con)nuously and spontaneously during DNA replica)on
- Mutagens can increase probability of muta)ons occurring:
a) Ionising radia)on from X rays can break DNA strands, which are then altered in the repair process
b) Deamina)ng chemicals can alter the chemical structure of bases, conver)ng one into another
c) Methyl/ethyl groups can be added to bases -> incorrect pairing
d) Viruses can insert sec)ons of viral DNA into DNA of cells
- Most muta)ons have no effect on us:
a) Don’t change the polypep)de, or only alter slightly, so the structure/func)on is unchanged (as the
gene)c code is degenerate so oMen different triplets code for the same amino acid)
b) Muta)on occurs in non-coding region so have no effect on the sequence
- However, a muta)on can lead to a change in the polypep)de the gene codes for: via inser)on, dele)on or
subs)tu)on of one or more nucleo)des
Inser2on of nucleo2des
- Nucleo)de is randomly inserted into the DNA sequence -> changes the amino
acid that would have been coded for by the original base triplet, as it creates a
new, different triplet of bases
- Knock-on effect by changing the triplets further on in the DNA sequence ->
frameshiM muta)on
- Could drama)cally change the amino acid sequence produced from the gene,
hence the polypep)de’s ability to func)on
Dele2on of nucleo2des
- Nucleo)de is randomly deleted from the DNA sequence, also changes the amino acid coded for
- Knock-on effect by changing the groups of 3 bases further on in the DNA sequence -> frameshiM muta)on
- Could also drama)cally change the amino acid sequence produced from the gene, hence the polypep)de’s
ability to func)on
Subs2tu2on of nucleo2des
- Base in DNA sequence randomly swapped for another one, 3 forms
- Only changes the amino acid for the triplet in which the muta)on occurs, no
knock-on effect
1. Silent muta)ons: don’t alter amino acid sequence (degenerate gene)c code)
2. Missense muta)ons: alters single amino acid in polypep)de (sickle cell anaemia)
3. Nonsense muta)ons: creates a premature stop codon (stops transla)on of
mRNA) -> polypep)de is incomplete -> affects structure + func)on of protein
Effect on Polypep2des
Beneficial muta2ons
- Some result in a significantly altered polypep)de with a different shape, hence altering ability of protein to
perform its func)on e.g. ac)ve site ability to bind to substrate/structural protein may lose strength
- Some)mes this may result in a beneficial altered characteris)c for the organism
- E.g. produc)on of melanin: Early humans in Africa had dark skin with high conc of melanin to provide
protec)on from harmful UV radia)on -> as humans moved north/south to lower sunlight intensi)es, pale
skin synthesises vitamin D more easily so muta)ons occurred to decrease melanin produc)on -> pale-
skinned people had a selec)ve advantage as lack of vitamin D causes health problems e.g. rickets
Harmful muta2ons
- Altering the polypep)de can cause a changed characteris)c with harmful effects in an organism
- Many gene)c diseases e.g. haemophilia, sickle cell anaemia, cys)c fibrosis: caused by dele)on muta)on of
3 nucleo)des in gene coding for CFTR protein -> causes lung and pancrea)c problems from extremely thick
mucus
, Neutral muta2ons
- No selec)ve advantage/disadvantage to the organism because:
a) Muta)on doesn’t alter polypep)de
b) Muta)ons only alters polypep)de slightly -> no change to structure/func)on
c) Muta)on alters structure/func)on of polypep)de but the resul)ng difference in the characteris)c has
no par)cular advantage/disadvantage to the organism
6.1.2 Gene Control
- Nucleus of every cell contains the same genes, but not every gene is expressed in every cell, and not all of
these genes are expressed all the )me
- Regulatory mechanisms ensure the correct genes are expressed in the correct cell at the correct )me
- 3 main types: transcrip)onal (during transcrip)on), post-transcrip)onal (aMer transcrip)on) and post-
transla)onal (aMer transla)on)
- Regulatory mechanisms are controlled by regulatory genes
Structural and regulatory genes
- Structural genes code for a protein with a func)on within a cell (enzymes/membrane carrier/hormone)
- Regulatory genes code for a protein that control the expression of structural genes and their levels of
protein produc)on, some)mes controlling several structural genes simultaneously
6.1.3 Gene Control: Lac Operon
- If structural genes are controlled during transcrip)on, gene control occurs at transcrip)onal level -> lac
operon is a regulatory mechanism at this level
The lac operon
- Structural genes in prokaryotes can form an operon: cluster of genes controlled by same promoter
- The ‘lac’ operon is found in bacteria and control produc)on of enzyme lactase (B-galactosidase) which
breaks down substrate lactose to be used as energy source, and 2 other structural proteins
- Lactase is an inducible enzyme -> only synthesised when lactose is present to prevent waste of
energy/materials
Structure of the lac operon
1. Promoter (region of DNA required for transcrip)on) for structural genes
2. Operator (segment of DNA to which repressor binds to inhibit transcrip)on)
3. Structural gene lacZ that codes for lactase
4. Structural gene lacY that codes for permease (allows lactase into the cell)
5. Structural gene lacA that codes for transacetylase
- Located to the leM (before) the lac operon on bacterium DNA is:
1. Promoter for regulatory gene,
2. Regulatory gene lacl that codes for lac repressor protein
- Lac repressor protein has 2 binding sites -> allowing it to bind to a) the operator in the lac operon and b) to
lactose (the effector molecule)
a) When it binds to the operator it prevents the transcrip)on of the structural DNA as RNA polymerase
cant acach to the promoter
b) When it binds to lactose the shape of the repressor protein distorts -> it can’t bind to the operator
When lactose is absent
1. Regulatory gene transcribed and translated to produce lac
repressor protein
2. Lac repressor protein binds to the operator region upstream of lacZ
3. Due to presence of repressor protein, RNA polymerase cant bind to
the promoter region
4. Transcrip)on of structural gene does not occur
5. No lactase enzyme is synthesised
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