MEiosis
6.1, 6.2, 6.6
6.1 – Chromosomes &
Meiosis
• Key Concept:
• Gametes have half the number of chromosomes that
body cells have.
You have somatic
cells and gametes.
• Somatic Cells:
• Are body cells
• Make up all cells in body except for
egg and sperm cells
• DNA not passed on to children
• Gametes:
• Are egg or sperm cells
• DNA passed on
to children
Your cells have autosomes
and sex chromosomes.
• Human somatic cells have 23 pairs of
chromosomes (46 total)
• (1) Autosomes: pairs 1 – 22; carry
genes not related to the sex of
an organism
• (2) Homologous chromosomes: pair of
• chromosomes; one from each parent; carry the same
genes but may have a different form of the gene
(example: one gene for brown eyes and one gene for blue
eyes)
• (3) Sex chromosomes: pair 23;
determines the sex of an animal;
control the development of
sexual characteristics (Females XX, Males XY)
Stop & Review
• Another name for body cells
• Somatic cells
• Another name for sperm & egg cells
• Gametes
• The number of pairs of chromosomes in somatic cells
• 23
• The number of individual chromosomes in somatic cells
• 46
• Chromosome pairs 1-22
• Autosomes
• Chromosome pair 23
• Sex chromosomes
Somatic cells are diploid;
gametes are haploid.
• Diploid (2n)
• Has two copies of each
chromosome (1 from
mother & 1 from father)
• 44 autosomes, 2 sex
chromosomes
• Somatic cells are diploid
• Produced by mitosis
• Haploid (1n)
• Has one copy of each
chromosome
• 22 autosomes, 1 sex
chromosome
• Gametes are haploid
• Produced by meiosis
• Chromosome number must be maintained in animals.
• Many plants have more than two copies of each
chromosome (can be tetraploid [4n] or hexaploid [6n])
• Mitosis and meiosis are types of nuclear division that
make different types of cells.
• Mitosis makes identical diploid cells.
• Meiosis makes different haploid cells from diploid cells.
Stop & Review
• The number of copies of each chromosome in a diploid cell
• 2
• The number of chromosomes in a human diploid cell
• 46
• The number of copies of each chromosome in a haploid cell
• 1
• The number of chromosomes in a human haploid cell
• 23
• This process produces genetically identical diploid cells
• Mitosis
• This process produces genetically unique haploid cells
• Meiosis
6.2 – Process of meiosis
• Key Concept:
• During meiosis, diploid cells undergo two cell divisions
that result in haploid cells.
Cells go through two
rounds of division in meiosis.
• Meiosis reduces chromosome number and creates
genetic diversity.
• Homologous chromosomes (sometimes called homologues)
• Pair of chromosomes
• Inherit one from each parent
• Carry same genes but code for different traits (different versions
of the gene)
• Separate during Meiosis I
• Sister chromatids
•
•
•
•
Duplicates of each other
Each half of a duplicated chromosome
Attached together at the centromere
Separate in Meiosis II
Stop & Review
• These separate during Meiosis I
• Homologous chromosomes
• These separate during Meiosis II
• Sister chromatids
Meiosis I
• Occurs after DNA has been replicated (copied)
• Divides homologous chromosomes in four phases.
Meiosis I
• (1) Prophase I
• Chromosomes condense
• Homologous chromosomes pair up
• Nuclear envelope (membrane)
breaks down
• Spindle fibers form
• Crossing over occurs
Meiosis I
• (2) Metaphase I
• Homologous chromosomes are lined up along the middle
of the cell (along the cell equator) by spindle fibers
Meiosis I
• (3) Anaphase I
• Homologous chromosomes move apart to opposite sides
of the cell
• Sister chromatids remain attached
Meiosis I
• (4) Telophase I & Cytokinesis
• Spindle fibers fall apart
• Nuclear membranes reform
• Cytoplasm splits
Meiosis II
• Divides sister chromatids in four phases.
• DNA is not replicated between Meiosis I and Meiosis II.
Meiosis II
• (5) Prophase II
• Nuclear envelope (membrane) breaks down
• Spindle fibers form
Meiosis II
• (6) Metaphase II
• Spindle fibers line chromosomes up along the middle of
the cell
Meiosis II
• (7) Anaphase II
• Sister chromatids are pulled apart to opposite sides of the
cell
Meiosis II
• (8) Telophase II & Cytokinesis
•
•
•
•
Nuclear membranes form around chromosomes
Chromosomes begin to uncoil
Spindle fibers fall apart
Cytoplasm splits
Mitosis Vs. Meiosis
Mitosis
Meiosis
• One cell division
• Two cell divisions
• Homologous chromosomes
do not pair up
• Homologous chromosomes
pair up (Metaphase I)
• Results in 2 diploid cells
• Results in 4 haploid cells
• Daughter cells are identical
to parent cell
• Daughter cells are unique
•
Stop & Review –
Mitosis
Or
Meiosis?
This involves two rounds of cell divisions
• Meiosis
• This produces 4 genetically unique haploid daughter cells
• Meiosis
• This results in daughter cells that are identical to the parent
cell
• Mitosis
• Homologous chromosomes pair up during this process
• Meiosis
• This results in 2 genetically identical daughter cells
• Mitosis
• This involves one round of cell division
• Mitosis
Haploid cells develop
into mature gametes.
• Gametogenesis is the production of gametes.
• Gametogenesis differs between males and females.
• Sperm (spermatogenesis)
• Become streamlined and motile (able to move)
• Primary contribution to embryo is DNA only
• Males produce over 250 million sperm per day
• Egg (oogenesis)
• Contribute DNA, cytoplasm, and organelles to the embryo
• During meiosis, the egg gets most of the contents, the other
3 cells become polar bodies
• Females produce a few hundred eggs over a lifetime
6.6 – Meiosis & Genetic
Variation
• Key Concept:
• Independent assortment and crossing over during meiosis
result in genetic diversity.
Sexual reproduction creates
unique combinations of genes.
• Fertilization
• Random
• Increases unique combinations of genes
• In humans, the chance of getting any one combination of
chromosomes from any one set of parents is one out of
223 x 223 (which is one out of over 70 trillion combinations)
Sexual reproduction creates
unique combinations of genes.
• Independent assortment of chromosomes
• Homologous chromosomes line up randomly along the
cell equator
• Increases the number of unique combinations of genes
• In human cells, about 223 (8 million) different
combinations could result
Sexual reproduction creates
unique combinations of genes.
• Crossing over
• Exchange of chromosome segments between homologous
chromosomes
• Increases genetic diversity
• Occurs during Prophase I of Meiosis I
• Results in new combinations of genes (chromosomes have a
combination of genes from each parent)
http://www.youtube.com/watch?v=5x_Rp1mwotQ
Genetic linkage
• Chromosomes contain many genes.
• The farther apart two genes are located on a chromosome, the more
likely they are to be separated by crossing over
• Genetic linkage: genes located close to each other on the same
chromosome tend to be inherited together