Meiosis

Asexual reproduction yields two identical organisms (with exception of mutations)

Sexual reproduction merges the genes of the two parents

Result: two versions of each chromosome◦ Called homologous chromosomes

Organism is diploid (2n=46) All somatic cells are diploid

Creation of Gametes

All can be lined up based on size, creates a karyotype

First 22 are autosomes Chromosome 23 is the sex chromosome Haploid (n=23) sex cells merge to create a

zygote in sexual reproduction Meiosis creates these sex cells (gametes) The overall process is the sexual life cycle

Homologous chromosomes

Chromosomes replicate

Meiosis I

Meiosis II

Homologous chromosomes separate

Sister chromatids separate

Interphase I of Meiosis

Similar to mitosis, chromatin replicates Centrioles replicate, proteins form around to

make the centrosomes◦ Also called the microtubule organizing centre◦ Collectively, with microtubules, is called mitotic

spindle

Interphase I

Longest part of meiosis Chromatin condenses, chromosomes

become visible◦ Homologous chromosomes come pair up, forming

a tetrad – called synapsis◦ The chromatids can overlap, called a chiasmata

All other features are similar to mitosis

Prophase I

Almost identical to metaphase in mitosis, with the tetrad exception

Metaphase I

Sister chromatids remain attached, tetrads separate (cells are now haploid)

Anaphase I

Homologous chromosomes are at opposite poles of cell◦ Each end is now haploid

Cytokinesis takes place simultaneously Some cells proceed to interphase II, others

directly to prophase II No cells replicate chromatin again

Telophase I

Spindle forms again, chromosomes move towards equator

Prophase II

Closely resembling mitosis, metaphase plate is formed

Metaphase II

Sister chromatids finally separate

Anaphase II

Mitotic spindle breaks down, nucleus reforms around two daughter nuclei

Telophase II

Some of the processes can go awry If anaphase I or II does not proceed

correctly, the chromosomes may not be shared evenly

This is called nondisjunction More of a problem if it occurs during meiosis

I Can take place in mitosis, but not as much

of a concern

Errors

An abnormal chromosome number is called aneuploidy

If faulty gamete succeeds in fertilization, the offspring with have health consequences

If a gamate with an extra chromosome fertilizes, the zygote is trisomic (2n+1)

If the gamete has a chromosome missing, the zygote is monosomic (2n-1)

Ex. Extra chromosome 21 – Down syndrome

If the extra chromosome is an X in a male (XXY), called Klinefelter syndrome

The individual will have male sex organs, but the testes will be abnormally small, and he will be sterile

As well, feminine characteristics such as breast enlargement is common

Final example is Turner syndrome Example of monosomy X Individuals are classified X0, and are female However, sex organs do not mature at

adolescence and secondary sex characteristics fail to develop

Also possible for anaphase I or II to completely fail and all chromosomes move to one cell

Referred to as polyploidy Could have three chromosome sets

(triploidy, 3n) or four (tetraploidy, 4n)

Polyploidy