Meiosis IB Topic 4.2 – Page 93

Meiosis A form of cell division which results in gametes (sex cells) Sexual reproduction Leads to genetic variation Fertilization = fusion of sperm and egg Superficially resembles mitosis, but there are some fundamental differences

Unique Characteristics Each new cell has only half the number of chromosomes that a typical cell in that organism has. For example, humans have 46 chromosomes in their cells, but in the sperm and egg cells, there are only 23 chromosomes Cells which contain half the chromosome number are called haploid cells Cells with the full chromosome number are called diploid cells.

Are you listening … Identify a cell in your body that is diploid. Identify a cell in your body that is haploid. What is the difference between the two?

What do we get at the end of meiosis? The cells formed are called daughter cells Meiosis generates: 4 haploid daughter cells that are genetically different from one another Unique mix of half of the genetic information of the parent cells (half from mom and half from dad) Quick review: how does this differ from mitosis?

Meiosis is also called … Reduction division Why? Because the # of chromosomes has been reduced This reduction is necessary because during sexual reproduction, each parent contributes 50% of the genetic material 23 chromosome egg (n) + 23 chromosome sperm (n) = 46 chromosome (2n) fertilized egg What do you think might happen if human cells do not have 46 chromosomes?

Homologous Chromosomes In a diploid human cell, the 46 chromosomes can be grouped into 23 pairs of chromosomes called homologous chromosomes Homologous means similar in shape and size and centromere position They also carry the same genes One from mom and one from dad

Homologous Chromosomes Although the pair carry the same genes, they are not identical because the alleles for the genes from each parent could be different

The phases of meiosis Step-by-step process by which a diploid parent cell produces 4 haploid daughter cells Before the steps begin, DNA replication allows the cells to make a complete copy of it genetic information during interphase Sound familiar?

The phases of meiosis In order to produce four cells, the parent cell must divide two times 1st mitotic division = 2 cells 2nd mitotic division = 4 cells

Crossing Over – Another Fundamental Difference Occurs during prophase 1 of meiosis Also called synapsis All of the chromatids of the two homologous chromosomes become tightly associated Combined pair is a bivalent or a tetrad The maternal and paternal chromosomes exchange corresponding sections of DNA Once complete, new combinations of alleles have been created The process by which offspring possess a combination of alleles different form that of either parent is called recombination

Why crossing over? Crossing over allows DNA from a person’s maternal chromosomes to mix with the DNA from the paternal chromosomes Recombinant chromatids that end up in the sperm or egg cells are a mosaic of the parent’s original chromatids.

Chiasma An X-shaped structure formed between non-sister chromatids during prophase 1 Physical manifestation of crossing over Persist through metaphase 1

Steps of meiosis Good animation http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

Meiosis and genetic variation The random orientation of chromosomes at metaphase 1 leads to variation within offspring The 4 haploid cells produced are genetically different from each other b/c: 1. independent assortment of maternal and paternal homologous chromosomes: The way the bivalents line up at the equator in meiosis 1 is entirely random In humans, there are 23 pairs of chromosomes, so the # of possible combinations is 2^23 (8 million) 2. crossing over of segments: New combinations of genes on the chromosomes Unimaginable degree of variation