Meiosis Varia%on - The art of crossing over
Vocabulary You Need to Know! Chroma%d Chroma%n Sister Chroma%ds Chromosomes Homologous Chromosomes Tetrad Centromere Daughter Cell Parent Cell Haploid Diploid Crossing Over Gene%c Recombina%on Independent Assortment
Diploid Diploid vs Haploid 2 copies of each chromosome All our body cells are diploid 2n Haploid one copy of each chromosome Our sex cells (sperm, egg) are haploid 1n *n= number of copies of chromosome*
MEIOSIS AND CROSSING OVER Chromosomes are matched in homologous pairs Each synapsis is made up of 2 pairs of sister chroma8ds This matched set of 4 chroma8ds is called a tetrad Chromosomes Centromere Sister chromatids
Crossing over further increases gene%c variability Crossing over is the exchange of corresponding segments between two non- sister chroma8ds of homologous chromosomes Gene8c recombina8on results from crossing over during prophase I of meiosis This increases varia8on further
How crossing over leads to gene8c recombina8on Nonsister chroma8ds break in two at the same spot The 2 broken chroma8ds join together in a new way Coat-color genes 1 2 3 4 Eye-color genes Tetrad (homologous pair of chromosomes in synapsis) Breakage of homologous chromatids Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types
A segment of one chroma8d has changed places with the equivalent segment of its nonsister homologue If there were no crossing over meiosis could only produce 2 types of gametes Coat-color genes 1 2 3 4 Eye-color genes Tetrad (homologous pair of chromosomes in synapsis) Breakage of homologous chromatids Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types
Increasing Gene%c Diversity During Meiosis I two steps are key in increasing gene%c diversity 1. Crossing over: During Prophase I Homologous chromosomes exchange pieces of DNA 2. Independent assortment: During Anaphase II Homologous chromosomes and/or sister chroma%ds are randomly assigned to each daughter cell
Crossing Over
Independent Assortment
Why Meiosis? Humans are DIPLOID: 23 pairs of chromosomes Sex cells are HAPLOID: contain 1 set of 23 chromosomes Fer%liza%on of two SEX CELLS (sperm and egg) create one ZYGOTE that is DIPLOID Sexual reproduc%on creates a zygote that is gene%cally different than both its parents
Why is Meiosis Important? First and foremost it allows for gene8c variability This is the most important part of Meiosis. You get chromosomes from both parents. This is what allows you to be different from your parents or siblings. Gene%c variability means that everything isn t a clone of the next It helps maintain the chromosome number in species Meiosis can play a part in evolu%on (via natural selec%on) THE MULE
Stages The stages look very similar to the stages of mitosis- a few differences: Meiosis I Homologous chromosomes pair (tetrad) and line up at equator Sister chroma%ds move to the SAME pole of the Tetrad cell
Meiosis II Stages Each daughter cell con%nues to divide to produce two haploid cells No replica%on of DNA between Meiosis I and Move me to reveal important informa%on! Meiosis II