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Meiosis

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Jean Battinieri

on 6 April 2016

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Transcript of Meiosis

Introduction to Heredity
the transmission of traits from one generation to the next
variation - offspring differ somewhat in appearance from parents and siblings
genetics - the scientific study of heredity and hereditary variation
genes - hereditary units with coded information - segments of DNA
DNA is a polymer made up of nucleotides
every living species has a characteristic number of chromosomes
46 chromosomes in humans
one chromosome contains hundreds or thousands of genes
where the genes are located on the chromosome are called a gene's locus or loci (plural)
comparision of sexual and asexual reproduction
asexual = a single individual is the sole parent and passes copies of all its genes to its offspring
ex single celled eukaryote reproduce via mitosis
hydra, a relataive to the jellyfish, is multicellulr and reproduces via budding
when an individual reproduces asexually it gives rise to a clone
clone = a group of genetically identical individuals
Sexual reproduction
when 2 parents produce offspring that have a unique combination of genes inherited from 2 parents
RESULTS IN GREATER VARIATION WITHIN A SPECIES THAN ASEXUAL REPRODUCTION
offspring of sexual reproduction vary genetically from their siblings and both parents
This variation is due to the behavior of the chromosomes during the sexual life cycle
Meiosis in sexual life cycles
a life cycle=the generation to generation sequence of stages in the reproductive history of an organism from conception to production of its own offspring
HUMAN LIFE CYCLE
each human somatic cell has 46 chromosomes
the chromosomes can be distinguished by their size and position of the centromere and when dye is added it gives it a distinctive banding pattern
karyotypes are the arrangement of homologous chromosomes
most chromosomes are autosomes but 1 pair are sex chromosomes
in humans the diploid number is 46 or 2n=46
The only cells in the human body NOT produced by mitosis are the gametes
sexually reproducing organisms carry out a process that halves the chromosome number in the gametes - compensating for the doubling that occurs at fertilization called MEIOSIS
Mitosis conserves chromosome number and Meiosis reduces it by half
Fertilization restores diploid number by combining 2 sets of chromosomes this also leads to GENETIC VARIATION
Meiosis
both
Mitosis
replicate chromosomes during interphase prior to starting
process of meiosis II the same as mitosis
starts diploid and goes haploid cell
2 divisions
reduces chromosomes number by half
creates 4 genetically different cells
starts diploid and stays diploid
1 division
keeps chromosome number constant
creates 2 genetically identical cells
G1
S phase
G2 phase
Prophase I
chromosomes condense
homologous chromosomes pair up = synapsis
tetrad forms
crossing over occurs - the exchange of genetic information between homologs
centrosomes move apart
nuclear membrane breaks down
spindle fibers form
prophase I of meiosis I can last for days or longer this typically takes more than 90% of the time spent in meiosis
Metaphase I
Tetrads line up in the middle
kinetochore fibers from one pole attaches to one chromosome of each pair and from the opposite pole attaches to its homologue
the way homologous chromosomes are arranged in the middle is the second way genetic variation can occur - maternal down one side and paternal down the other or a mixture of each on both sides
Anaphase I
The spindle fibers pull the tetrad apart and homologues move to opposite sides of the cell
sister chromatids are still attached at the centromere
Telophase I
one homolog of each of the homologous pairs is now at each side of the cell
nuclear membrane reforms
nucleolus re-forms
chromsomes return to chromatin
animal cells - cleavage furrow forms
plant cells cell plate forms
Cytokinesis I / Interkinesis
division of the cytoplasm
forms 2 daughter cells
these are haploid cells but the chromosomes are still duplicated
can be a resting stage but NO further DNA replication takes place
Prophase II
spindle fibers form
nuclear membrane breaks down
nucleolus disappears
chromosomes become visible
centrioles move to opposite sides in animal cells
Metaphase II
chromosomes move to the middle of the cell
kinetochore fibers attach at the centromere
Anaphase II
sister chromatids separate
chromatids move to opposite sides of the cell
Telophase II
nuclear membrane reforms
nucleolus appear
chromatids return to chromatin
spindle fibers disappear
cleavage furrow in animal cells
cell plate in plant cells
Cytokinesis II
final split of the cytoplasm in both cells
creates 4 genetically different haploid cells
Differences between male and female gametes
male gametes
there are 4 genetically different sperm cells created
each cell is equal in size and ability to fertilize an egg
females
there is an unequal split of cytoplasm when each cytokinesis occurs
there is only 1 actual egg cell created and 3 polar bodies
the one egg is viable and can be fertilized
the 3 polar bodies become waste
Interphase
Prophase I
Metaphase I
Anaphase I
Telophase I
Full transcript