4B Diversity, Classification and Variation

What are gametes? What forms when they meet and fertilise?

What are gametes? What forms when they meet and fertilise?

sex cells- sperm and eggs.

A zygote forms

What is significant about chromosome numbers in gametes and normal cells?

Gametes contain a haploid number of chromosomes (n) compared to normal cells which have a diploid number of chromosomes (2n).

How are gametes formed?

Meiosis-

• before meiosis begins, the DNA replicates so that there are two copies of each chromosome, called chromatids.

• the DNA condenses to form double-armed chromosomes each made from two sister chromatids, and the sister chromatids are joined by the centromere.

• In Meiosis 1, the chromosomes arrange themselves into homologous pairs, which are then separated, halving the chromosome number.

• In Meiosis 2, the pairs of sister chromatids that make up each chromosome are separated as the centromere is divided.

• Four haploid gametes that are genetically different are produced.

What are the two main events in Meiosis that lead to genetic variation?

1- crossing over of chromatids- this leads to 4 daughter cells containing chromatids with different alleles.

2- independent segregation of chromosomes- each homologous pair is made up from one maternal and one paternal chromosome. In Meiosis 1, it is completely random which chromosome from each pair ends up in which daughter cell, so the gametes produced have different combinations of those chromosomes.

What are chromosome mutations?

errors in mitosis cause variations, leading to inherited conditions as the errors are present in the gametes.

What is non-disjunction?

A type of chromosome mutation where the chromosomes do not separate properly.

What are gene mutations?

Changes to the base sequence of DNA

What is a substitution gene mutation?

One base is substituted for another, eg. ATGCCT becomes ATTCCT. This will not always to lead changes in the amino acid sequence due as the genetic code is degenerate.

What is a deletion gene mutation?

A base is deleted, eg. ATGCCT becomes ATCCT. This will always lead to changes in the amino acid sequence as there will be a shift in all the base triplets after the deletion as the number of bases changes.