Meiosis Gamete formation Key words Meiosis Prophase Metaphase

Meiosis Gamete formation

Key words… • Meiosis • Prophase • Metaphase • Anaphase • Telophase • Homologous pairs • Chiasmata/chiasma • Crossing over • Diploid • Haploid • Maternal chromosomes • Paternal chromosomes

Revision The human chromosome complement is 46 chromosomes This means humans have 23 pairs of chromosomes There are 22 matching pairs of autosomes There is one pair of matching sex chromosomes Males have an X and a Y chromosome, whereas females have two X chromosomes Matching pairs of chromosomes are referred to has homologous pairs, therefore males have 22 homologous pairs whereas females have 23 pairs

Male karyogram showing 22 homologous autosomes and one pair of sex chromosomes (XY) female karyogram showing 22 homologous autosomes and one pair of homologous sex chromosomes (XX)

Haploid and Diploid Summary Normal body cell/zygote Contains 46 chromosomes or 23 pairs = diploid (2 n) Gametes (sex cells) Contains 23 chromosomes, half of normal body cells = haploid (n)

Meiosis is a cell division in which the number of chromosomes in a gamete mother cell (cell which produces gametes) is reduced from two sets (diploid) to a single set (haploid) http: //www. cellsalive. com/meiosis. htm

First Meiotic Division Chromosomes exist as single threads of information Chromosomes undergo DNA replication prior to cell division To increase the variation in genetic information in each cell, some sections of genetic information are swapped over between homologous chromosomes This point of crossing over is called chiasmata http: //www. johnkyrk. com/meiosis. html

Second Meiotic Division The chromosomes align at the equator of the cell once again Chromosomes are pulled to opposite poles Cell and nuclear division occurs The end product is four daughter cells, each containing the haploid number of chromosomes

Significance of the haploid Number Gametes produced at the end of meiosis must have the haploid number of chromosomes If they do not, the normal chromosome complement will not be achieved at fertilisation This can result in various genetic abnormalities or failure to produce viable offspring

Crossing-over Homologous pairs of chromosomes are in very close contact with each other This results in segments of genetic information being swapped with its homologous partner The point at which this crossing over occurs is called chiasmata

Crossing-over Exchange of chromosomal information in this way produces further variation in gametes Independent assortment and crossing over increase variation in gametes Gametes have a unique genotype, therefore a zygote is also unique as its genotype is very different to other zygotes

Independent Assortment The random alignment of homologous pairs of chromosomes is called independent assortment In this example, blue represents genetic information from an ovum and yellow represents genetic information from a sperm As a result of the random nature in which chromosomes pair up with their homologous partner, four different gametes are produced Independent assortment ensures variation First Meiotic division Second Meiotic division Possible Gametes

Task 1 Meiosis animation of all stages Collect a cut out sheet to help you summarise the stages of meiosis I and II. Use pages 118 -119 from your textbook to help you write descriptions of each stage.

Title: Significance of meiosis Learning question: Why is meiosis important? Starter List three difference between meiosis and mitosis

Key words…reminder • Meiosis • Prophase • Metaphase • Anaphase • Telophase • Homologous pairs • Chiasmata/chiasma • Crossing over • Diploid • Haploid • Maternal chromosomes • Paternal chromosomes

Meiosis Is the special form of cell division used to produce gametes It has two functions: To form haploid cells with half the chromosome number To re-arrange the chromosomes with a novel combination of genes

Meiosis comprises of two successive divisions without DNA replication in between The stages of these divisions are on your worksheet… Either draw or cut and stick the corresponding picture in the boxes

Meiosis Rearrange the following steps of meiosis Separation of chromatids Formation of bivalents 4 Pairing by homologous chromosomes 2 Production of haploid cells 1 Chiasma formation and exchange between chromatids 5 3

Genetic Variation Meiosis and sexual reproduction allows for genetic variation This allows species to adapt to their environments through evolution There are three sources of genetic variation in sexual reproduction Can you name them? Independent Assortment Crossing Over Random Fertilisation

Independent Assortment Happens in metaphase I, when the bivalent line up on the equator Each bivalent is made up of two chromosomes The chromosomes can line up in any order on the equator resulting in different possible combinations

Independent Assortment In the sample below, n=2 and there are 4 possible gametes In humans n=23 so there are over 8 million possible different gametes!

Crossing Over This occurs in prophase I While the two homologous chromosomes are joined in a bivalent, bits of one chromosome are swapped (crossed over) with the corresponding bits of the other chromosome

Crossing Over There always equal amounts crossed over so chromosomes stay the same length Crossing over means that the maternal and paternal alleles can be mixed

Random Fertilisation This takes place when two gametes fuse to form a zygote Each gamete has a unique combination of genes and any of the numerous male gametes can fertilise the egg

Gene Mutation DNA is very stable but bases can change during DNA replication Most mutation have no phenotypic effect, these are called silent mutations

Variation Below is an outline of causes of variation As well as genetic causes, individuals are influenced by the environment Genotype + environment = phenotype

Discontinuous Variation Are distinct, separate peaks/bars Are distinct categories to place individuals Tends to have no overlap between categories Are controlled by a small number of genes Are largely unaffected by the environment

Continuous Variation Is a continuous range of values so get a smooth curve Are no distinct categories to place individuals Tends to have overlap between categories Are controlled by a large number of genes Are significantly affected by the environment

Sex Determination Females produce eggs which carry the X chromosome. Sperm carry either X or Y Males therefore determine the sex of the child Females = XX Males = XY

Sex Determination It is though that Y chromosomes dislike acid conditions within the female reproductive systems Y chromosomes are smaller and will therefore float in a sample of semen if centrifuged Clinics can use this information to select sex

sson Key words…new to this le • Allele • Locus • Random assortment • Genetic re-assortment • Segregation • Random mutations