Facts About Meiosis • Daughter cells contain half the number of chromosomes as the original cell (haploid or 1n). • Produces gametes • Egg and sperm • Occurs in the testes in males • spermatogenesis • Occurs in the ovaries in females • Oogenesis
Why Do we Need Meiosis? • It is the fundamental basis of sexual reproduction • Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote • Promotes genetic diversity
Fertilization: Putting it all together 2n = 6 1n =3
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 Occurs in Interphase
SisterChromatids Same genes, Same alleles (traits) Homologs Same genes, different alleles (traits) A Replicated Chromosome Gene X Homologs separate in meiosis I and therefore different alleles (traits) separate.
from mom from dad child too much! meiosis reduces genetic content Meiosis Forms Haploid Gametes • Meiosis must reduce the chromosome number by half • Fertilization then restores the 2n number The right number!
Meiosis I MeiosisII Meiosis: Two Part Cell Division Sister chromatids separate Homologs separate Diploid Diploid Haploid
Spindle fibers Nucleus Nuclear envelope Meiosis I: Reduction Division Early Prophase I (Chromosome number doubled) Late Prophase I Metaphase I Anaphase I Telophase I (diploid)
Prophase I • Late prophase • Chromosomes condense. • Spindle forms. • Nuclear envelope fragments. Early prophase Homologs pair. Crossing over occurs
Tetrads Form in Prophase I Homologous chromosomes(each with sister chromatids) Join to form a TETRAD CalledSynapsis
Crossing-Over • Homologous chromosomes in a tetrad cross over each other • Pieces of chromosomes or genes are exchanged • Produces genetic recombination in the offspring
Metaphase I Homologous pairs of chromosomes 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.
Sister chromatids carry identical genetic information. Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Meiosis II Only one homolog of each chromosome is present in the cell. Gene X
Meiosis II: Reducing Chromosome Number Prophase II Metaphase II Anaphase II Telophase II 4 haploid cells
Prophase II Nuclear envelope fragments. Spindle forms.
Metaphase II Chromosomes align along equator of cell.
Anaphase II Equator Pole Sister chromatids separate and move to opposite poles.
Telophase II Nuclear envelope assembles. Chromosomes uncoil. Spindle disappears. Cytokinesis divides cell into two.
Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome
Spermatogenesis • Occurs in the testes • Two divisions produce 4 spermatids • Spermatids mature into sperm • Men produce about 250,000,000 sperm per day
Oogenesis • Occurs in the ovaries • Two divisions produce 3 polar bodies that die and 1 egg • Polar bodies die because of unequal division of cytoplasm • Immature egg called oocyte • Starting at puberty, one oocyte matures into an ovum (egg) every 28 days
First polar body may divide (haploid) a Polar bodies die X a a X X a X X Mitosis Meiosis I Meiosis II A (if fertilization occurs) Oogonium (diploid) A Primary oocyte (diploid) X Ovum (egg) X A Mature egg Secondary oocyte (haploid) A X Second polar body (haploid) Oogenesis
WWhat are the four steps to the cell cycle? In which step does the nucleus content divide? What is it called when you divide the cytoplasm? T/F All cell division happens at the same rate. What limits the size of cell? In mitosis, cytokinesis produces two identical>>>>
TheThe process of organizing and condensing long strands of DNA into compact form takes place when? What are the group of proteins called that organizes and condenses the long strands of DNA into tight coils? During which phase of mitosis do sister chromatids separate from each other? What stage completes the cell cycle? A liver cell is an example of which type of cell?
How many chromosomes are in a human gamete? What does it mean to be homologous? When are haploid gametes produced? At fertilization, what happens to the nucleus? What is the result of Meiosis I? What happens to sister chromatids in meiosis II? Gametogenesis is the term for????
Does the egg or the sperm carry the organelles for the fertilized cell? What is replication? What is the main function of DNA polymerase? What does it mean to be semi-conservative? What is the ‘Central Dogma’? What is the RNA strand for GTAGTCA?
What is the term for a three nucleotide sequence that codes for an amino acid? How many amino acids are used to make the proteins in the human body? A tRNA that carries the amino acid methionine pairs with what type of codon? What is the name for converting the mRNA into a polypeptide? Where does translation occur?