CHAPTER 11 BIOLOGY 11TH EDITION RAVEN

Meiosis

Responsible for the variation we see in offspring

Reduces the number of chromosomes.

Sexual life cycle

• Made up of meiosis and fertilization, or syngamy (syn- "together")

◦ Meiosis reduces the number of chromosomes

Diploid cells

◦ Somatic (non reproductive) cells of adults have 2 sets of chromosomes

Haploid cells

◦ Gametes (eggs and sperm) have only 1 set of chromosomes

Sexual reproduction

• Reproduction that involves an alternation of meiosis and fertilization

Diploid state dominates

In most animal cells.

Zygote first undergoes mitosis to produce diploid cells.

Later in the life cycle, germ line cells undergo meiosis to produce haploid gametes

Germ line cells

Diploid cells that undergo meiosis to form gametes, reproductive cells, haploid cells, sperm and egg.

Meiosis includes 2 rounds

Meiosis 1 &2. Each has prophase metaphase, anaphase, and telophase stages.

synapsis

During prophase 1 after replication.

Homologous chromosomes become closely associated.

Includes formation of synaptonermal complexes

Complexes

Tetras or bivalents

Genetic recombination (crossing over)

Occurs when homologues are paired during prophase 1

chiasmata

Site of crossing over

reduction division

First meiotic division.

Homologous chromosomes are pulled to opposite poles.

Results in daughter cells that contain one homologue from each chromosome pair

NO DNA DIVISION BETWEEN MEIOTIC DIVISIONS

second meiotic division

separates sister chromatids

Process of Meiosis

*meiotic cells have an interphase period that is similar to mitosis with G1, S, and G2 phases

*after interphase, germ-line cells enter meiosis I

*Meiosis I

-prophase I

-metaphase I

-anaphase I

-telophase I

*Meiosis II

-prophase II

-metaphase II

-anaphase II

-telophase II

Prophase 1

• Chromosomes coil tighter and become visible, nuclear envelope disappears, spindle forms

• Each chromosome composed of 2 sister chromatids

• Synapsids

◦ Homologues become closely associated

◦ Sister chromatids of each homologue are also joined by the cohesion complex in a process called sister chromatid cohesion

crossing over

• Genetic recombination between non sister chromatids

◦ Allows the homologues to exchange chromosomal material

• Alleles (alternate forms) of genes that were formerly on separate homologues can now be found on the same homologue

Metaphase 1

Paired homologues locked together following crossing over

• Microtubules from opposite Poles attatch to each homologue at their kinetochores

• Homologues are aligned at the metaphase plate side by side ]

• Orientation of each pair of homologues on the spindle is random

Anaphase 1

• Microtubules of the spindle shorten, pulling homologous chromosomes apart

◦ Chiasmata break

• Homologues are separated from each other and moved to opposite poles

◦ Sister chromatids remain attached to each other at their centromeres

• Each pole has a complete haploid set of chromosomes consisting of one member of each homologous pair

• Independent assortment of maternal and paternal chromosomes

◦ Due to random nature in which the chromosomes align during metaphase 1, each chromosome assorts independently from one another

Telophase 1

• Nuclear envelope re-forms around each daughter nucleus

• Sister chromatids are no longer identical because of crossing over (prophase 1)

• Cytokinesis may or may not occur after telophase 1

• Meiosis 2 occurs after an interval of variable length

• Human oocytes generally begin meiosis 2 directly after, but halt at metaphase 2 until fertilization

Meiosis 2

• Resembles a mitotic division (identical to mitosis)

• Prophase 2: nuclear envelopes dissolve and new spindle apparatus forms

• Metaphase 2: chromosomes align on metaphase plate

• Anaphase 2: sister chromatids are separated from each other

• Telophase 2: nuclear envelope re-forms around 4 sets of daughter chromosomes; cytokinesis follows

final result of meiosis

• Four cells containing haploid sets of chromosomes

• In animals, develop directly into gametes

• In plants, fungi, and many protists, divide mitotically

◦ Produce greater number of gametes

◦ Produce adults with varying numbers of chromosome sets

Errors in meiosis

Nondisjunction, aneuploid gametes, and most common cause of spontaneous abortion in humans

Nondisjunction

Failure of chromosomes to move to opposite poles during either meiotic division

Aneuploid gametes

gametes with missing or extra chromosomes