Meioooosis

Meiosis animation Meiosis Meiosis

Form of cell division where there are two successive rounds of cell division following DNA replication Produces haploid cells (n) Start with 46 double stranded chromosomes (2n) –After 1 st division - 23 double stranded chromosomes (n) –After 2nd division - 23 single stranded chromosomes (n) Occurs in our germ cells –cells that produce our gametes egg and sperm

-To bring two haploid gametes together to form a diploid zygote. -n (mom) + n (dad) = 2n (offspring) Why do we need meiosis? - It is the fundamental basis of sex What is the purpose of sex?

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 cell division forming gametes Goal: reduce genetic material by half Why? from momfrom dadchild meiosis reduces genetic content too much!

Meiosis: cell division in two parts Homologs separate Sister chromatids separate Result: one copy of each chromosome in a gamete. Haploid Diploid Meiosis I Meiosis II Haploid

Meiosis II : the equational division Prophase II (haploid) Metaphase II (haploid) Anaphase II (haploid) Telophase II (haploid) Four nonidentical haploid daughter cells

Called a tetrad

A replicated chromosome Homologs separate in meiosis I and therefore different alleles separate. homologs same genes maybe different alleles sister chromatids same genes same alleles Gene X

Spermatogenesis: sperm formation

Spermatogeneis Stem cells in testes divide mitotically to create a pool of spermatocytes. Meiosis produces four spermatids.

Oogenesis Oogonium (diploid) Mitosis Primary oocyte (diploid) Meiosis I Secondary oocyte (haploid) Meiosis II (if fertilization occurs) First polar body may divide (haploid) Polar bodies die Ovum (egg) Second polar body (haploid) a A X X a X A X a X a X Mature egg A X A X

Oogenesis: ovum formation One of four meiotic products becomes an ovum. The three remaining meiotic products are polar bodies.

Meiosis I : the reduction division Prophase I (early) (diploid) Prophase I (late) (diploid) Metaphase I (diploid) Anaphase I (diploid) Telophase I (diploid) Nucleus Spindle fibers Nuclear envelope

Prophase I Early prophase Homologs pair. Crossing over occurs. Late prophase Chromosomes condense. Spindle forms. Nuclear envelope fragments.

Metaphase I Homolog pairs align along the equator of the cell.

Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain Attached at their centromeres.

Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two. Have 2 cells, each with 46 chromosomes

Meiosis II Only one homolog of each chromosome is present in the cell. Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Sister chromatids carry identical genetic information. Gene X

Prophase II Nuclear envelope fragments. Spindle forms.

Metaphase II Chromosomes align along equator of cell.

Anaphase II Sister chromatids separate and move to opposite poles.

Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two.

Results of meiosis Gametes Four haploid cells One copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome

Fig 2.19

MitosisMeiosis Number of divisions 1 2 Number of daughter cells 24 Genetically identical? YesNo Chromosome #Same as parentHalf of parent WhereSomatic cellsGermline cells WhenThroughout lifeAt sexual maturity Role Growth and repair Sexual reproduction

Mitosis vs. Meiosis NOVA Online | 18 Ways to Make a Baby | How Cells Divide: Mitosis vs. Meiosis (Flash) NOVA Online | 18 Ways to Make a Baby | How Cells Divide: Mitosis vs. Meiosis (Flash)

Why do we have meiosis? - to generate haploid gametes - to make new combinations of genes -How? random (independent) assortment recombination

Independent assortment The homolog of one chromosome can be inherited with either homolog of a second chromosome.

Fig 2.20

Random assortment Since the combination of maternal and parental chromosomes received by a gamete is random. And we have 23 pairs of chromosomes The possible combinations in an egg or a sperm are = 8,388,608 combinations in an offspring 2 23 X 2 23 = 70,368,744,177,664 Result: Generates new combinations of genes (alleles) when the genes are located on different chromosomes.

Recombination Crossing-over - the physical exchange of chromosomal material between chromatids of homologous chromosomes. - Result: Generation of new combinations of genes (alleles) if the genes are located on the same chromosome.

Recombination (crossing over) Occurs in prophase of meiosis I Generates diversity Creates chromosomes with new combinations of alleles for genes A to F. a b c d e f A B C D E F A B C D E F a b c d e f

Recombination (crossing over) Occurs in prophase of meiosis I Generates diversity Letters denote genes Case denotes alleles Creates chromosomes with new combinations of alleles for genes A to F. A B C D E F a b c d e f c d e f A B a b C D E F

Recombination (crossing over) Occurs in prophase of meiosis I Generates diversity Letters denote genes Case denotes alleles Creates chromosomes with new combinations of alleles for genes A to F. A B C D E F a b c d e f c d e f A B a b C D E F

Gametogenesis - spermatogenesis - oogenesis

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.