4.3 Genetics diversity can arise as a result of mutation or during meiosis Flashcards
What is an allele
An alternative form of a gene
What are the names of the stages in meiosis
- Interphase 1+2
- Prophase 1+2
- Metaphase 1+2
- Anaphase 1+2
- Telophase 1+2 (+ cytokinesis 1+2)
What is a mutation
A change in the DNA base sequence (order of adenines, thymines, cytosines and guanines)
What does a cell look like before going through meiosis (the chromosomes)
- 2 copies of each chromosome
- Each chromosome consists of 1 chromatid
What is a point mutation and when does it occur
- A change in the base sequence of a gene which can cause a change in the polypeptide chain
- Caused by errors that occur during DNA replication
What happens during prophase 1
- Chromosomes condense and become visible
- Nuclear envelope disintegrates
- Spindle fibres form from centrioles and attach to chromosomes
- Crossing over may occur
What are the 3 main types of point mutation
- Substitution
- Deletion
- Addition
What happens during metaphase 1
- Spindle fibres attach to centromeres
- Homologous chromosome pairs align along the equator/ metaphase plate of the cell
What is a chromosome mutation and when does it occur
- A change in the number or structure of the chromosomes
- Caused by errors that occur during cell division
What happens during anaphase 1
- Spindle fibres contract and pull chromosomes towards centrioles
- Homologous chromosomes separated to opposite poles of the cell
What is substitution
One base is switched out for another which changes the codon.
What happens during telophase 1
- Nuclear membrane reforms
- Cytokinesis occurs to divide the cell
- Newly formed cells are diploid (2n)
What are 3 consequences of substitution
- ‘missense’: mutation codes for a single different amino acid
- ‘nonsense’: when the triplet codes for a ‘stop’ codon, so the polypeptide finishes at the point of mutation
- ‘silent’: the change has no affect on the amino acid coded for because the genetic code is degenerate
What happens during prophase 2
- Chromosomes condense and become visible
- Nuclear envelope disintegrates
- Spindle fibres project from centrioles and attach to centromeres
What is deletion
A base is deleted, causing all bases after to shift to the left.
What happens during metaphase 2
- Spindle fibres attach to centromeres
- Chromosomes align along the equator/metaphase plate of the cell
What is addition
A base is added, causing all bases after to shift to the right.
What happens during anaphase 2
- Spindle fibres contract and pull chromosomes towards centrioles
- Sister chromatids are separated at centromeres to opposite poles
What are the consequences of deletion and addition
- Deletion = left frameshift
- Addition = right frameshift
- The earlier in the DNA this happens, the greater the effect on the primary structure
- Very likely to result in a non-functional protein
What happens during telophase 2
- Nuclear membrane reforms
- Cytokinesis occurs to divide the cell
- Results in 4 genetically different haploid cells (gametes)
How can deletion be cancelled out
2 more left frameshifts or one right frameshift
What are 2 ways in which genetic variation is increased during meisosis
- Crossing over (of chromosomes)
- Independent segregation (of chromosomes)
How can addition be cancelled out
2 more right frameshifts or one left frameshift.
What is crossing over and what does it result in
- Homologous chromosomes pair up (end of prophase)
- Chromatids of homologous chromosomes cross over by forming a chiasma
- The chromatids exchange genetic material to produce a new combination of alleles
