Meiosis
Chapter 11 Continued
How can two guinea pigs (one brown & white, and one brown & black) reproduce and have a litter of multicolored guinea pigs?
I. Meiosis- cell division in which the daughter cells receive only half the number of chromosomes present in the parent cells (click to see animation of meiosis)
A. Homologous chromosomes- pairs of similar chromosomes (same size, shape, & carrying alleles [genes] for the same trait)
1. Humans have 46 chromosomes (22 pairs of homologous chromosomes & one pair of sex chromosomes)
2. Autosomes- are the chromosomes other than the sex chromosomes
ex) somatic or body cells
B. Diploid cells- contain the full # of chromosomes (2n)
C. Haploid cells- gametes that contain half the # of chromosomes (n); no homologous pairs
D. Gametes- specialized sex cells
1. female gametes = egg cells or ova (ovum is singular; egg cell)
2. male gamete = sperm cell
E. Fertilization- the fusion of sperm & ova nuclei to form a zygote
II. Stages of Meiosis- "reduction division" in specialized cells for reproduction. It is a process which prepares sex cells.
A. Prophase I
1. each chromosome has already replicated & consists of 2 chromatids
2. each chromosome lines up relatively close with its homologous chromosomes & become attached at their centromeres. This process is called SYNAPSIS.
a. synapsis results in 4 chromatids joining together to form a tetrad
3.while tetrads form, CROSSING OVERoccurs, which is the exchange of segments between chromatids
B. Metaphase I
1. centromeres are attached by the spindle fibers
2. tetrads line up at the equatorial plane
C. Anaphase I
1. DISJUNCTION- separation of tetrads into homologous chromosomes
2. Nondisjunction- is the failure to separate homologous chromosomes (very bad)
D. Telophase I
1. Cytokinesis occurs forming daughter cells
2. nuclear membrane reappears around the nuclei
3. no further replication of chromosomes takes place, but the remainder of the division is exactly like mitosis.
E. Prophase II [both daughter cells divide]
F. Metaphase II [homologous chromosome line up on the equator]
G. Anaphase II [homologous chromosomes separate]
H. Telophase II [4 different haploid cells are produced, these (n) are gametes]
A comparison (must see)chart between Mitosis and Meiosis
|
Mitosis |
Meiosis |
|
1. occurs in growth & asexual reproducing organisms |
1. occurs in gonads; the production of gametes |
|
2. homologous chromosomes do not pair up in prophase. There is no exchange of parts between homologous chromosomes. |
2. Homologous chromosomes pair up during prophase I, crossing-over occurs. |
|
3. involves one cell division |
3. involves 2 cell division |
|
4. two daughter cells receive the same # of chromosomes as the parent cell |
4. the 4 daughter cells have half the # of chromosomes of the original cell |
|
5. mitosis maintains the chromosome # |
5. meiosis reduces the chromosome # in half |
III. Sexual Reproduction
A. Advantages
1. offspring are not identical to either parent
2. variations which increases adaptability in a changing environment
3. individuals are able to move into new environments
4. by making population more varied helps to ensure survival of the species
IV. Sexual Reproduction in Simple Organisms- CONJUGATION (permits the recombination of hereditary material, to form new variety of organisms)
A. Spirogyra- the flow of cell material between 2 cells through a conjugation tube and the formation of a zygospore (protective wall)
B. Paramecium- formation of a protoplasmic bridge between 2 and the exchange of material. Don't have sexes, but different mating types positive/negative isogametes.
C. Bacteria
V. Sexual Reproduction in Animals
A. Gonads- specialized organs that produce gametes
1. Ovaries- female which produce ova (egg cells)
2. Testis- male which produce sperm cells
B. Hermaphrodism - contain both ovaries and testes
C. Gametogenesis - production of gametes
1. oogenesis- production of ova in the ovaries
a. meiotic division that produces 3 polar bodies and one mature ovum
2. spermatogenesis- production of sperm in the testes
a. meiotic division that produces 4 sperm cells
D. Comparison of egg and sperm
1. egg cells contain yolk, and are larger than sperm
2. sperm is motile, eggs are sessile
E. Fertilization - fusion of sperm and egg nuclei to form a zygote (Animation of ovulation, fertilization, & implantation)
1. diploid (2n) chromosomes restored
2. external fertilization - takes place outside of body - more eggs are produced
3. internal fertilization - takes place inside body, less eggs are produced
F. Parthenogenesis- the development of an unfertilized egg into an adult animal
return to Mr. Via's syllabus
