MeiosisMeiosis

STS BiologySTS Biology

Unit 4 GeneticsUnit 4 Genetics

Chapter 11Chapter 11

Chromosome Number Chromosome Number

Chromosome NumberChromosome Number Eukaryotic cells have pairs of Eukaryotic cells have pairs of

homologous chromosomeshomologous chromosomes Cells with a Cells with a doubledouble set of chromosomes set of chromosomes

are are diploid (2n)diploid (2n) Cells with Cells with oneone set of chromosomes are set of chromosomes are

haploid (n)haploid (n)

Humans Humans bodybody cells cells are diploid (2N)are diploid (2N) 2N = 462N = 46

GametesGametes - sex cells - sex cells Males – spermMales – sperm Females – eggsFemales – eggs

Gametes are Gametes are HAPLOIDHAPLOID N = 23N = 23

Fertilization restores Fertilization restores the diploid state (N + the diploid state (N + N = 2N)N = 2N)

ZygoteZygote - a fertilized egg (2N)

Meiosis…bottom Meiosis…bottom lineline

Process to make haploid Process to make haploid gametesgametes

Chromosome # is cut in Chromosome # is cut in halfhalf 2N 2N N N

Occurs in ovaries & testesOccurs in ovaries & testes TWO cell divisionsTWO cell divisions Increases genetic Increases genetic

variationvariation

Prior to Prior to Meiosis….INTERPHASEMeiosis….INTERPHASE

What happens during Interphase???What happens during Interphase??? DNA REPLICATION!!!DNA REPLICATION!!!

2N

2N

Meiosis IMeiosis I• Prophase I – homologous chromosomes form tetrads & crossing over occurs• Metaphase I – tetrads line up along the middle

• Anaphase I – homologous chromosomes separate• Telophase I – two nuclei form

• Cytokinesis I – cells separate

2N

N N

Crossing OverCrossing Over Occurs in Occurs in Prophase IProphase I Chromosomes exchange portions of their Chromosomes exchange portions of their

chromatids chromatids Increases genetic variationIncreases genetic variation

Meiosis IIMeiosis II• Prophase II – spindle forms, prepares for division

• DNA replication DOES NOT occur between Meiosis I & II

• Metaphase II – chromosomes align along the center• Anaphase II – sister chromatids separate

• Telophase II – nuclei reform; cytokinesis

N N

N N NN Looks A LOT like MITOSIS!

Meiosis in Men vs. Meiosis in Men vs. WomenWomen

Men: 4 sperm Men: 4 sperm are madeare made

Women: only Women: only 1 egg is made1 egg is made

Meiosis vs. MitosisMeiosis vs. Mitosis

Independent AssortmentIndependent Assortment Mendel’s Law of Independent AssortmentMendel’s Law of Independent Assortment Alleles (form of a gene) separate independently of Alleles (form of a gene) separate independently of

each othereach other Leads to the formation of MANY different gametesLeads to the formation of MANY different gametes

Meiosis SummaryMeiosis Summary Chromosome number is cut in half!Chromosome number is cut in half! Produces haploid gamete cells – egg and spermProduces haploid gamete cells – egg and sperm Tetrads form during Prophase ITetrads form during Prophase I

Crossing over occurs in tetrad formation – Crossing over occurs in tetrad formation – leads to increased genetic variationleads to increased genetic variation

Two cell divisions, but DNA does not replicate Two cell divisions, but DNA does not replicate between divisionsbetween divisions

Results in 4 sperm cells in men and 1 egg in Results in 4 sperm cells in men and 1 egg in womenwomen

Which chromosomes wind up in which sex Which chromosomes wind up in which sex cells? It’s random…due to independent cells? It’s random…due to independent assortment!!assortment!!