4.3 Gene mutation Flashcards
What is a mutation?
any change to the quantity or base sequence of the DNA of an organism
What is a gene mutation?
Any change to one or more nucleotide base or a change in the sequenced of the bases, in DNA
What are the 3 main ways the DNA sequence can change?
- addition of a base
- substitution of a base
- deletion of a base
What are the causes of a mutation?
- can be spontaneous during DNA replication
- Some things increase the rate of mutations: UV radiation, ionising radiation, some chemicals and some viruses
Why might a mutation not lead to a change in the amino acid sequence?
- genetic code is degenerate so mutation may end up coding for same amino acid as the original triplet
- mutation may occur in intron
What is substitution mutation?
When a nucleotide in the DNA sequence is replaced by another. This is more likely to be a quiet mutation, meaning no changes occur in the amino acid sequence
What is a deletion mutation?
When a nucleotide in the DNA sequence is lost. This is more likely to be harmful and significant, as it leads to a frame shift which means the entire amino acid sequence will be different
What is a mutagenic agent?
Factors that increase the rate of a gene mutation. X-rays, UV light, gamma rays, certain chemicals
What is a polyploidy chromosome mutation?
When an individual has three or more sets of chromosomes instead of two
What is chromosome non-disjunction?
When chromosomes fail to separate correctly in meiosis, resulting in gametes with one more or less chromosome than normal
What is meiosis?
A form of cell division that produces 4 genetically different haploid cells known as gametes
How does meiosis differ from mitosis?
-Meiosis produces 4 genetically different cells with half the number of chromosomes as the parent cell
- Mitosis produces 2 genetically identical cells with the same number of chromosomes as the parent cells
What happens in meiosis 1?
- homologous chromosomes pair up and their chromatids wrap around each other
- Crossing over (exchange of sections of genetic material) occurs in chiasmata
- Cells divide into two. Homologous chromosomes separate randomly. Each cell contains either maternal or paternal copy
What happens during meiosis 2?
- the chromatids move apart
- each cell divides again, producing haploid cells
Give 2 ways meiosis produces genetic variation.
- crossing over during meiosis 1
- independent assortment of homologous chromosomes and sister chromatids result in new combinations of alleles
What is independent segregation?
- happens in meiosis 1 when each chromosome lines up alongside its homologous partner
- the chromosomes can line up in any order resulting in different possible combinations
What is crossing over?
- the chromatids of each pair become twisted around one another
- during this twisting process tensions are created and portions of the chromatids break off
- these broken portions might then rejoin with the chromatids of its homologous partner
- Usually it is the equivalent portions of homologous chromosomes that are exchanged
- in this way new genetic combinations of maternal and paternal alleles are produced
- Chromatids cross over one another many times and so the process is known as crossing over. The broken portions of chromatid recombine with another chromatid, so this process is called recombination
How can independent segregation be calculated?
2^n where n = the number of pairs of homologous chromosomes