NOTES FOR BIOLOGY 1001

SECTION 005

Spring 2005

DR. STEVEN POMARICO

CHAPTER 10

MEIOSIS

>>>>>>A Comparison of asexual and sexual reproduction

---Asexual reproduction is the type of reproduction involving only one parent. Produces genetically identical offspring.

-budding

-binary fission

-no meiosis

---Sexual reproduction is the type of reproduction which involves two parents and produces an offspring with unique combinations of the genes inherited from the two parents.

ASEXUAL REPRODUCTION	SEXUAL REPRODUCTION
Only one parent	Two parents produce offspring
Single parent passes on all its genes	Each parent passes on half its genes
Rare genetic differences in offspring is the result of DNA changes, or mutations	Greater genetic variation with offspring being genetically different from their parents and siblings

---Mutation is a rare change in the DNA of the genes that creates genetic diversity.

Meiosis and fertilization result in alterations between the haploid and diploid condition.

>>>>>>Meiosis reduces chromosome number from diploid to haploid

-Chromosome replication preceded meiosis (just like mitosis)

-replication is followed by two consecutive cell divisions.

Meiosis I and Meiosis II

-divisions produce 4 daughter cells instead of 2

-these daughter cells have half the number of chromosomes.

STAGES OF MEIOSIS

Unique events occur during meiosis I

1 - Interphase

-Chromosome replication

2 - Meiosis I - First division

a) Prophase I (90% of meiosis)

-Chromosomes condense

-Homologous chromosomes undergo pairing up

-Sister chromatids are linked at centromeres, nonsister chromatids are linked at chiasmata

-Genetic crossing over occurs (see fig 12-4)

-centrioles move apart

-nucleoli disappear

-nuclear envelope disappears

-the spindle forms but it’s different than in mitosis

Meiosis kinetochores attached to same centrosome

Mitosis kinetochores attached to different centrosomes

---Chiasmata are the regions of paired homologous chromosomes where chromatids have exchanged genetic material because of a crossing over

b) Metaphase I

-chromosome pairs align on metaphase plate

-kinetochores of sister chromatids face same pole

-centromeres of homologues face opposite poles.

c) Anaphase I

-spindle microtubules interact with kinetochore fibers

-homologous chromosomes move toward opposite poles

-sister chromatids remain attached (this is different than mitosis)

d) Telophase I and cytokinesis

-chromosomes arrive at poles

-each pole has haploid set, but each chromosome has sister chromatids

-nuclear envelope may or may not reform

-cell divides

-NO FURTHER REPLICATION OF CHROMOSOMES.

3 - Meiosis II - Second division (very similar to mitosis)

a) Prophase II

-if nuclear membrane reformed at telophase I, then the nuclear membrane disperses

b) Metaphase II

-Chromosomes align at metaphase plate as in mitosis

c) Anaphase II

-Sister chromatids separate and move toward opposite poles

d) Telophase II and cytokinesis

-nuclei form at opposite poles

-cell division occurs

-four haploid daughter cells are the final result

>Variations of the sexual life cycle (See fig 10.7)

There are three types of variation of the sexual life cycles:

1. Diploid life cycle

-found in animals, including humans

2. Haploid life cycle

-most fungi, and some protists

3. Alternation of generations

-plants and some algae

-has a multicellular diploid stage which undergoes meiosis to produce spores.

-also has a multicellular haploid stage which produces gametes by mitosis.

>>>>>>Sexual life cycles promote genetic variation among offspring

-orientation of homologous pairs is random

=> 50% chance daughter cell will get a certain chromosome of the homologous pair

-there are 2^C possible combinations, where C is the number of chromosomes/gamete

=> in humans 2²³ = about 8.4 million possible combinations