Meiosis and Sexual Life Cycles

(SEE ATTACHED DOCUMENT FOR A PRINTER-FRIENDLY VERSION)

Meiosis and Sexual Life Cycles

Genes = segments of DNA that code for basic units of heredity

-offspring acquire genes from parents by inheriting chromosomes

Types of reproduction – asexual and sexual

-Asexual – produces clones (genetically identical). Single parent. Little variation in population, only through mutations. Fast and energy efficient.

Ex: budding, binary fission.

-Sexual – meiosis produces gametes (sex cells). 2 parents: male/female. Lots of variation/diversity. Slower and energy consumptive.

Ex: humans/trees

Chromosomes

-Somatic (body) cell: 2n = 46 chromosomes

-each pair of homologous chromosomes includes 1 chromosome from each parent.

-Autosomes: 22 pairs of chromosomes that do not determine sex

-Sex chromosomes: X and Y. Females = XX. Males = XY

-Gametes (n = 23): 22 autosomes + 1 sex chromosome

-egg = 22 + X

-sperm = 22 + X **or** 22 + Y

Homologous chromosomes in a somatic cell

-sister chromatids – one duplicated chromosome – each dup is a sister chromatid

-pairs of homologous chromosomes – one from each set (mom and dad)

-nonsister chromatids in a homologous pair (one dup mom and one dup dad)

-centromere – point of attachment

Karyotype = picture of an organism’s complete set of chromosomes. Arranged from largest to smallest pair

-used to determine genetic abnormalities

ex: some cancer cells have abnormal #s of chromosomes

HeLa Cells

-oldest and most commonly used human cell line

-cervical cancer cells taken from Henrietta Lacks (d. 1951)

-immortal cells – do not die after a few divisions – active version of telomerase

-used in research: develop vaccine for polio. Cancer, AIDS, virus radiation research

-Estimated that cells produced in culture exceeded # cells in Henrietta’s body

-Ethical concerns: cells were harvested w/o the patient’s consent. “discarded tissues can be commercialized” and sold for profit. Her genome was published in 2013 w/o her family’s consent

-book: “The Immortal Life of Henrietta Lacks” by Rebecca Skloot

Life Cycle: reproductive history of an organism from conception to production of own offspring

-fertilization and meiosis alternate in sexual life cycles

-meiosis – cell division that reduces # of chromosomes (2n à n), creates gametes

-fertilization – combine gametes (sperm + egg)

-fertilized egg = zygote (2n)

-zygote divides by mitosis to make multicellular diploid organism

There are Varieties of Sexual Life Cycles

-Animals: diploid multicellular organisms that produce haploid gametes by meiosis

-Alternation of Generations – plants and some algae

-sporophyte (2n): makes haploid spores by meiosis

-spore à gametophyte by mitosis

-gametophyte (n): makes haploid gametes by mitosis

Meiosis = reduction division

-cells divide twice

-result = 4 daughter cells, each w half as many chromosomes as parent cell

Meiosis I (1^st division)

-Interphase: chromosomes replicated

-Prophase I: synapsis – homologous chromosomes pair up. Form tetrad – 4 sister chromatids together. Crossing over at the chiasmata occurs.

-Metaphase I: tetrads line up

-Anaphase I: pairs of homologous chromosomes separate. Sister chromatids still attached by centromere.

-Telophase I and cytokinesis: haploid set of chromosomes in each cell. Each chromosome = 2 sister chromatids. In some species chromatin and nucleolus reform.

Meiosis II (2^nd division)

-Prophase II: no interphase, no crossing over, spindle forms.

-Metaphase II: chromosomes line up.

-Anaphase II: sister chromatids separate

-Telophase II: 4 haploid cells, nuclei repair, each daughter cell genetically unique

Events Unique to Meiosis I (not in mitosis)

1. Prophase I: synapsis and crossing over

2. Metaphase I: pairs of homologous chromosomes line up on the metaphase plate

3. Anaphase I: homologous pairs separate à sister chromatids still attached at centromere

Sources of genetic variation:

1. Crossing over: exchange genetic material. Recombinant chromosomes

2. Independent assortment of chromosomes: random assortment of homologous pairs in metaphase I

3. Random fertilization: any sperm + any egg (8 million X 8 million = 64 trillion possible combinations)

Mitosis vs. Meiosis (both are divisions of the cell nucleus)

-Mitosis: somatic cells. 1 division. 2 diploid daughter cells. Clones. From zygote to death. Purpose is growth and repair. No synapsis. No crossing over.

-Meiosis: gametes. 2 divisions. 4 haploid daughter cells. Genetically different. Females before birth follicles are formed, mature ova released beginning puberty. Purpose is reproduction.

COMPARISON: Mitosis vs. Meiosis

DNA replication

Mitosis – occurs during interphase before mitosis begins

Meiosis – occurs during interphase before meiosis begins

Number of Divisions

Mitosis – ONE. Includes P M A T

Meiosis – TWO. Each includes P M A T.

Synapsis of homologous chromosomes

Mitosis – NO

Meiosis – occurs during prophase I along w crossing over between non-sister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion.

Number of daughter cells and genetic composition

Mitosis – TWO. Each diploid (2n) and genetically identical to the parent cell

Meiosis – FOUR. Each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other.

Role in animal body

Mitosis – enables multicellular adult to arise from zygote; produces cells for growth, repair, and in some species, asexual reproduction.

Meiosis – produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes