B9 GENETIC DIVERSITY- MUTATIONS AND MEIOSIS

What’re the 2 types of mutation?

• gene mutation

• chromosome mutation

What’s gene mutation?

a change in the base sequence of chromosomes

What’s chromosome mutation?

changes in the number of whole chromosomes

How’re mutations arisen spontaneously?

• gene mutations arise during DNA replication

• chromosome mutations arise during meiosis

What increases the risk of gene mutations?

• mutagenic agents

• X-rays, Gamma rays, UV radiation, asbestos, viruses

What is a gene mutation?

due to a change in the base sequence of the DNA of a gene, causing the sequence of amino acids to change, so a different protein is produced

What’re the 2 types of gene mutation?

• base substitution

• base deletion

What occurs during base substitution?

involves swapping of one nucleotide for another during DNA replication

Why can base substitution cause a change in the proteins?

during transcription the complementary codon is changed, so during translation the wrong amino acid is inserted into the polypeptide

Why does a base substitution not always cause a change in protein?

the genetic code is degenerate, so not all base substitutions cause a change in the sequence of encoded amino acids

What occurs during base deletion?

occurs when one or more nucleotides are removed from the DNA sequence during DNA replication, causing a shift in the whole sequence of bases and all the base triplets are changed after this point (known as a Frame shift)

What’re possible consequences of gene mutations?

• doesn’t change the amino acid sequence due to the degenerate genetic code

• a base substitution may not affect the function of the protein if it has the same properties as the original one

• a base substitution may affect the bonding, changing the protein structures meaning it is non-functioning

• a base deletion may result in an entirely different amino acid sequence

What is meiosis?

produces 4 haploid daughter cells that’re genetically different from each other

Why is meiosis important?

• produces haploid gametes, meaning upon fertilisation the zygote will be diploid, ensuring the chromosome number remains constant from one generation to the next

What is the S phase of meiosis?

the DNA is replicateed

How is genetic variation introduced between the daughter cells?

• independent segregation of homologous chromosomes

• crossing over between sister chromatids

What occurs during the independent segregation of homologous chromosomes?

• during metaphase of the first division, the homologous chromosomes attach to the spindle fibres and they randomly line up

What occurs during crossing over of non-sister chromatids in a homologous pair?

the homologous chromosomes attach to one another and when they’re moved the chromatids may break at the attachment point. enzymes then reattach the fragments, however the fragments may be attached to the other chromatid, meaning the alleles may be different

What occurs during meiosis 1?

• meiosis 1 results in the production of 2, diploid daughter cells (same stages of mitosis)

• crossing over

What occurs during meiosis 2?

1. nuclear envelope breaks down again and chromosomes condense

2. chromosomes line up in the equator and bind to spindle fibres at the centromeres where the chromatids of each chromosome are independently assorted

3. spindle fibres pull the chromatids to opposite ends of the cell, so the centromere divides

4. 2 nuclear envelopes develop to from 2 haploid nuclei

Why is genetic variation important?

produces new combinations of characteristics meaning that the species is more likely to adapt to changes in the environment

What is non-disjunction?

the homologous pair fails to separate during anaphase and one daughter cell ends up with both homologous chromosomes while the other daughter cell ends up with non

What does polyploidy mean?

an increase in entire sets of chromosomes

What does polysomy mean?

the gain of one single chromosome