Purines- less pure things- adenine, guanine
Pyrimidines- need to CUT non-pure things- cytosine, uracil, thymine
Repair processes during S phase while replicating
2 reasons for variation-
● Mutation- change in DNA sequence of gene spontaneously or induced exposure to radiation/
chemicals called mutagents that increase rate at which mutations occur. It is source of
evolutionary change.
● Recombination- second level of variation. Cellular processes causing the alleles of different
genes to be grouped together new genes.
Point mutation- change in single base pair or small no. of adjacent base pairs of DNA. Some point
mutations show no phenotypic change, as other sites than mutated gene can imitate their function or not
in a function part.
Consequences of mutation- 1) If degeneracy is maintained. 2) If there is a translational stop codon.
Eg of mutation in non-coding regions-
● Splice site mutation- insertion and deletion into mRNA that may not be in frame originally.
● Regulatory region mutation- region essential for RNA polymerase and regulating proteins.
Mutation in these regions affect the amount of protein produced instead of its structure. It could
completely inactive that protein if binding site for RNA pol is disrupted.
Spontaneous mutation-
Is mutation spontaneous (random process)-
● Q- does mutation occur as a physiological change in response to selecting agent or variants
present at low frequency in a population where mutants are produced spontaneously and
randomly?
- Experiment- When E. coli infected by phage T1. Some are resistant to phages- genuine mutants.
- The no. of resistant colonies highly varied or “fluctuated” per culture.
- The early mutations gave the higher numbers of resistant cells because the mutant cells had time
to produce many resistant descendants. The later mutations produced fewer resistant cells.
, Purines- less pure things- adenine, guanine
Pyrimidines- need to CUT non-pure things- cytosine, uracil, thymine
Repair processes during S phase while replicating
Fluctuation test hypothesis-
Spontaneous mutation mechanism-
1) Error in DNA replication-
● highly accurate process, but mistakes can occur when millions bases are copied.
● DNA can be easily altered, the cellular environment itself can damage it.
- Transitions- Happens when the wrong base pair is formed, in tautomeric or ionised (more
frequent) form. Eg- A-C. Corrected by DNA pol III or other repair system, otherwise remains.
- Transversion- energetically unfavourable as pairing of purine-pyrimidine as a particular dimension
that can be disrupted by this mutation.
- Frame mutations- indel of bases making them indivisible by 3, no. in codon.
Replication slippage- newly synthesised or template strand slips (certain regions prone
to slip as have many repeated sequences that can be wrongly replicated) → certain
extra bases loop out→ loop stabilised by repeated sequences present→ due to loop
the bases added or deleted.
Pyrimidines- need to CUT non-pure things- cytosine, uracil, thymine
Repair processes during S phase while replicating
2 reasons for variation-
● Mutation- change in DNA sequence of gene spontaneously or induced exposure to radiation/
chemicals called mutagents that increase rate at which mutations occur. It is source of
evolutionary change.
● Recombination- second level of variation. Cellular processes causing the alleles of different
genes to be grouped together new genes.
Point mutation- change in single base pair or small no. of adjacent base pairs of DNA. Some point
mutations show no phenotypic change, as other sites than mutated gene can imitate their function or not
in a function part.
Consequences of mutation- 1) If degeneracy is maintained. 2) If there is a translational stop codon.
Eg of mutation in non-coding regions-
● Splice site mutation- insertion and deletion into mRNA that may not be in frame originally.
● Regulatory region mutation- region essential for RNA polymerase and regulating proteins.
Mutation in these regions affect the amount of protein produced instead of its structure. It could
completely inactive that protein if binding site for RNA pol is disrupted.
Spontaneous mutation-
Is mutation spontaneous (random process)-
● Q- does mutation occur as a physiological change in response to selecting agent or variants
present at low frequency in a population where mutants are produced spontaneously and
randomly?
- Experiment- When E. coli infected by phage T1. Some are resistant to phages- genuine mutants.
- The no. of resistant colonies highly varied or “fluctuated” per culture.
- The early mutations gave the higher numbers of resistant cells because the mutant cells had time
to produce many resistant descendants. The later mutations produced fewer resistant cells.
, Purines- less pure things- adenine, guanine
Pyrimidines- need to CUT non-pure things- cytosine, uracil, thymine
Repair processes during S phase while replicating
Fluctuation test hypothesis-
Spontaneous mutation mechanism-
1) Error in DNA replication-
● highly accurate process, but mistakes can occur when millions bases are copied.
● DNA can be easily altered, the cellular environment itself can damage it.
- Transitions- Happens when the wrong base pair is formed, in tautomeric or ionised (more
frequent) form. Eg- A-C. Corrected by DNA pol III or other repair system, otherwise remains.
- Transversion- energetically unfavourable as pairing of purine-pyrimidine as a particular dimension
that can be disrupted by this mutation.
- Frame mutations- indel of bases making them indivisible by 3, no. in codon.
Replication slippage- newly synthesised or template strand slips (certain regions prone
to slip as have many repeated sequences that can be wrongly replicated) → certain
extra bases loop out→ loop stabilised by repeated sequences present→ due to loop
the bases added or deleted.
