meiosis - weebly · 2019-11-06 · gametogenesis • is the formation of sex cells in meiosis. •...
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Stages of Meiosis, Gametogenesis, Sex Determination, & Abnormalities.
MEIOSIS
Stages and Steps of Meiosis Interphase • Same as in Mitosis • 3 stages – G1, S, G2 • DNA replication and cell growth
Meiosis I Prophase I • Nuclear membrane begins to dissolve • Homologous chromosomes (made up of sister chromatids) match up and
intertwine in a process called Synapsis • Crossing over occurs which is the exchange/mixing of genetic information
between two non-sister chromatids • Crossing over results in chromosome recombination
Metaphase I • The pair of homologous chromosomes, each containing 2 sister
chromatids, is called a tetrad • The tetrads move toward the centre of the cell and align along the
metaphase plate. • Independent assortment occurs, which is the RANDOM arrangement of
the homologous chromosome along the metaphase plate
Anaphase I • The homologous chromosomes separate towards opposite poles of the
cell • Only 1 chromosome from each homologous pair will be found in the new
daughter cells
Telophase I • Division of cytoplasm and organelles • Nuclear membrane reappears and reforms as the cell begins to divide into
two cells *Cytokinesis • Ends in two haploid cells • Unlike mitosis, the chromosomes are NOT IDENTICAL (due to crossing
over)
Meiosis II *** the cell does not undergo interphase, it jumps directly into Meiosis II
Prophase II • DNA replication did not occur as interphase is skipped • One set of chromosomes still contain sister chromatids, HOWEVER they
are not identical due to crossing over that occurred in Prophase I • Nuclear membrane dissolves
Metaphase II • Chromosomes line up in a single file along the cell’s metaphase plate • Sister chromatids remain attached by their centromere
Anaphase II • Chromatids are pulled apart and pulled to opposite poles of the cell • The nuclear membranes begins to reform around the chromatids
Telophase II • Division of cytoplasm and organelles • Chromosomes unwind, nuclear membrane reforms and the cell begins to
divide into two cells *Cytokinesis • End with four haploid cells called gametes
Gametogenesis • Is the formation of sex cells in meiosis. • In animals it takes place in the testes and ovaries. • Produced by meiosis, sex cells contain the haploid number of
chromosomes. • The production of sperm is called spermatogenesis • The production of egg cells (ova) is called oogenesis.
The Differences Oogenesis • Cytoplasm is not equally divided between cells during meiosis. • One daughter gets cell most of the cytoplasm, which becomes a single
ovum (eggs cell) • The other 3 remaining cells are called polar bodies, which die.
Spermatogenesis • Cytoplasm is equally divided during both stages of meiosis • Results in 4 equally sized sperm cells • Streamlined for maximum motility
Karyotypes & Sex Chromosomes • Karyotype is a picture of chromosomes that are arranged by number, size, or
some other characteristic Sex Chromosomes • Found in most eukaryotic organisms • In most mammals - matching pair of homologous chromosomes in females and partially matching in males • In males one chromosome is smaller than the other
• The larger of the two is a homologous chromosomes to that in the female
Sex Determination • The larger type of sex chromosome is the X chromosome • The smaller type is the Y chromosome • X & Y chromosomes contain matching regions
• Allows for synapsis and act as homologous pair = meiosis • Non sex chromosomes are called autosomes
• XX = Female XY = Male • XX/XY is know as the sex-determination system
• You always get an X from the Mom, therefore dad determines the sex • If you get an X from dad = female. Get a Y from dad = male
• Not all animals use this sex-determination cycle
Abnormalities Meiosis • Can contribute to genetic disorders and infertility.
• Random meiotic errors, disease, and environmental factors Entire chromosomes can be lost or gained in a rare error during meiosis • Usually a result of non-disjunction
Non-disjunction • The failure of homologous chromosomes to separate during meiosis • Results in one daughter cell with an extra chromosome and one daughter
cell missing a chromosome • Not able to function normally
• Can be found via preparation of a karyotype
Outcomes of Non-disjunction Errors • Trisomy – Means 3 homologous chromosomes instead of a homologous
pair • Monosomy – Means 1 chromosome instead of a homologous pair
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