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Transcript
Mitosis v Meiosis Types of Cell Divisions Replication of chromosomes Replication is the process of duplicating a chromosome Occurs prior to division Replicated copies are called sister chromatids Held together at centromere Meiosis – A Source of Distinction Why do you share some but not all characters of each parent? What are the rules of this sharing game? At one level, the answers lie in meiosis. Meiosis –cell division - each cell divides twice - occurs rapidly - produces haploid cells (n) - (start - 46 double stranded chromosomes (2n) After 1 division - 23 double stranded chromosomes (n) After 2nd division - 23 single stranded chromosomes (n)) - occurs in our germ cells (remember latin? ) – cells that produce our gametes - egg and sperm Meiosis – A Source of Distinction Meiosis does two things - 1) Halves the number of chromosomes This is a good idea if you’re going to combine two cells to make a new organism. In meiosis, one diploid cells produces four haploid cells. 2) Meiosis mixes the forms of each gene that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over. Genetic diversity is important for the evolution of populations and species. Meiosis a cell division forming gametes Goal: reduce genetic material by half Why? from mom from dad child too much! meiosis reduces genetic content “Putting It All Together” - Fertilization Meiosis: cell division in two parts Sister chromatids separate Homologs separate Meiosis I (reduction division) Meiosis II (equational division) Diploid Haploid Haploid Result: one copy of each chromosome in a gamete. One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment is the random separation of homologous chromosomes. This lead to VARIETY! Crossing-Over Crossing-Over: when 2 duplicated chromosomes line up very close to one another during Prophase I and share genetic info. More Variety!!! Recombination (crossing over) Occurs in prophase of meiosis I A A B B C a a b C b c c d d Generates diversity D D E F E F e f •Creates chromosomes with new combinations of alleles for genes A to F. e f Recombination (crossing over) Occurs in prophase of meiosis I A a B b C C c c Generates diversity D D Letters denote genes Case denotes alleles E F d E F d e f •Creates chromosomes with new combinations of alleles for genes A to F. e f Recombination (crossing over) Occurs in prophase of meiosis I Generates diversity a A B b C D Letters denote genes Case denotes alleles E F A a B b C D E F c c d d e f •Creates chromosomes with new combinations of alleles for genes A to F. e f Meiosis Following a Single Chromosome Pair Crossing over Oogenesis – A Path of Meiosis in Humans Woman are less busy in meiosis than men – meiosis produces only a few hundred mature eggs over a lifetime. Spermatogenesis – Another Path of Meiosis in Humans Men are busy – meiosis produces roughly 250,000,000 sperm per day. Boy or Girl? The Y Chromosome “Decides” Y chromosome X chromosome Boy or Girl? The Y Chromosome “Decides” A. Sexual Reproduction versus Asexual reproduction great genetic variety versus no genetic variety 1. Asexual repro. needs only one parent 2. Sexual repro. needs two sex cells a. Sperm--male sex cell b. Egg--female sex cell Fertilization Fertilization is the joining of sperm and ovum. • Meiosis II in the ovum is completed at the time of fertilization forming one ovum and one polar body. • Following fertilization, chemical reactions occur preventing additional sperm from entering the ovum. Mitosis Meiosis 2 Number of divisions 1 Number of daughter cells 2 4 Yes No Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Growth and repair Sexual reproduction Genetically identical? Chromosome # Role What Meiosis is About Meiosis allows the creation of unique individuals through sexual reproduction.