EW Title Meiosis 1. Define the term gene. 2. What is the difference between sexual and asexual reproduction?

EW Title Meiosis

1. Define the term gene.

2. What is the difference between sexual and asexual reproduction?

Chapter 13~

Meiosis and Sexual Life Cycles

Heredity Heredity: the transmission of traits

from one generation to the next Variation: Inherited differences

among individuals of the same species.

Genetics: The study of heredity and hereditary variations.

Gene: A segment of DNA that codes for a particular trait

Locus: Location on a chromosome for a particular gene (It is consistent throughout a species).

Karyotype An ordered image of chromosomes

starting with the longest chromosome.

LE 13-4

Key

Maternal set ofchromosomes (n = 3)

2n = 6

Paternal set ofchromosomes (n = 3)

Two sister chromatidsof one replicatedchromosomes

Two nonsister chromatids in a homologous pair

Pair of homologouschromosomes(one from each set)

Centromere

Heredity Asexual reproduction: clones Sexual reproduction: variation Human life cycle:

• 23 pairs of homologous chromosomes (46);

• 1 pair of sex and 22 pairs of autosomes ]• gametes are haploid (1N)/ all

other cells are diploid (2N);

•fertilization (syngamy) results in a zygote

Meiosis: cell division to produce haploid gametes

Important Terms to Know

Autosomes: – Chromosomes that do not

determine the sex of an organism (1-22).

Diploid Cells:– Cells that have 2 sets of

chromosomes Haploid Cells: Cells that have 1 set of

chromosomes

Animal Life Cycle

Plants/some algae

Alternation of generations: 2N sporophyte, by meiosis, produces 1N spores; spore divides by mitosis to generate a 1N gametophyte; gametes then made by mitosis which then fertilize into 2N sporophyte

Fungi/some algae

meiosis produces 1N cells that divide by mitosis to produce 1N adults (gametes by mitosis)

Meiosis

Preceded by chromosome replication, but is followed by 2 cell divisions (Meiosis I & Meiosis II)

4 daughter cells; 1/2 chromosome number (1N); variation

Meiosis

Preceded by chromosome replication, but is followed by 2 cell divisions (Meiosis I & Meiosis II)

4 daughter cells; 1/2 chromosome number (1N); variation

Prophase I Prophase I typically occupies more than 90% of the time

required for meiosis Chromosomes begin to condense In synapsis, homologous chromosomes loosely pair up,

aligned gene by gene In crossing over, nonsister chromatids exchange DNA

segments Each pair of chromosomes forms a tetrad, a group of

four chromatids Each tetrad usually has one or more chiasmata, X-

shaped regions where crossing over occurred

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Sisterchromatids

Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Microtubuleattached tokinetochore

Tetrad

MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes (red and blue) pair andexchange segments; 2n = 6in this example

Pairs of homologouschromosomes split up

Tetrads line up

Metaphase I At metaphase I, tetrads line up at the

metaphase plate, with one chromosome facing each pole

Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad

Microtubules from the other pole are attached to the kinetochore of the other chromosome

LE 13-8ab

Sisterchromatids

Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Microtubuleattached tokinetochore

Tetrad

MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes (red and blue) pair andexchange segments; 2n = 6in this example

Pairs of homologouschromosomes split up

Tetrads line up

Anaphase I

In anaphase I, pairs of homologous chromosomes separate

One chromosome moves toward each pole, guided by the spindle apparatus

Sister chromatids remain attached at the centromere and move as one unit toward the pole

LE 13-8ab

Sisterchromatids

Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Microtubuleattached tokinetochore

Tetrad

MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes (red and blue) pair andexchange segments; 2n = 6in this example

Pairs of homologouschromosomes split up

Tetrads line up

Telophase I and Cytokinesis In the beginning of telophase I, each half of the

cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids

Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms

No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

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Cleavagefurrow

MEIOSIS II: Separates sister chromatids

PROPHASE II METAPHASE II ANAPHASE IITELOPHASE I AND

CYTOKINESISTELOPHASE II AND

CYTOKINESIS

Sister chromatidsseparate

Haploid daughter cellsforming

Two haploid cellsform; chromosomesare still double

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Prophase II

Meiosis II is very similar to mitosis In prophase II, a spindle apparatus forms In late prophase II (not shown in the art),

chromosomes (each still composed of two chromatids) move toward the metaphase plate

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Cleavagefurrow

MEIOSIS II: Separates sister chromatids

PROPHASE II METAPHASE II ANAPHASE IITELOPHASE I AND

CYTOKINESISTELOPHASE II AND

CYTOKINESIS

Sister chromatidsseparate

Haploid daughter cellsforming

Two haploid cellsform; chromosomesare still double

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Metaphase II

At metaphase II, the sister chromatids are arranged at the metaphase plate

Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical

The kinetochores of sister chromatids attach to microtubules extending from opposite poles

Animation: Metaphase II

LE 13-8b

Cleavagefurrow

MEIOSIS II: Separates sister chromatids

PROPHASE II METAPHASE II ANAPHASE IITELOPHASE I AND

CYTOKINESISTELOPHASE II AND

CYTOKINESIS

Sister chromatidsseparate

Haploid daughter cellsforming

Two haploid cellsform; chromosomesare still double

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Anaphase II

At anaphase II, the sister chromatids separate

The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

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Cleavagefurrow

MEIOSIS II: Separates sister chromatids

PROPHASE II METAPHASE II ANAPHASE IITELOPHASE I AND

CYTOKINESISTELOPHASE II AND

CYTOKINESIS

Sister chromatidsseparate

Haploid daughter cellsforming

Two haploid cellsform; chromosomesare still double

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Telophase II and Cytokinesis

In telophase II, the chromosomes arrive at opposite poles

Nuclei form, and the chromosomes begin decondensing

Cytokinesis separates the cytoplasm At the end of meiosis, there are four daughter

cells, each with a haploid set of unreplicated chromosomes

Each daughter cell is genetically distinct from the others and from the parent cell

LE 13-8b

Cleavagefurrow

MEIOSIS II: Separates sister chromatids

PROPHASE II METAPHASE II ANAPHASE IITELOPHASE I AND

CYTOKINESISTELOPHASE II AND

CYTOKINESIS

Sister chromatidsseparate

Haploid daughter cellsforming

Two haploid cellsform; chromosomesare still double

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome

sets, producing cells that are genetically identical to the parent cell

Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell

The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

Meiosis vs. Mitosis

Synapsis/tetrad/chiasmata (prophase I)

Homologous vs. individual chromosomes (metaphase I)

Sister chromatids do not separate (anaphase I)

Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes.

Origins of Genetic Variation, I

Independent assortment: homologous pair of chromosomes position and orient randomly (metaphase I) and nonidentical sister chromatids during meiosis II

Combinations possible: 2 ; with n the haploid number of the organism

n

Origins of Genetic Variation, II

Crossing over (prophase I): • the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes)

Random fertilization: • 1 sperm (1 of 8 million

possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!