1 meiosis and sexual life cycles living organisms are distinguished by their ability to reproduce...

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Meiosis and Sexual Life Cycles

• Living organisms are distinguished by their ability to reproduce their own kind

• Heredity– Is the transmission of traits from one generation to the

next• Variation

– Shows that offspring differ somewhat in appearance from parents and siblings

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Inheritance of Genes

• Genes are segments of DNA, units of heredity

• Offspring acquire genes from parents by inheriting chromosomes

• Genetics is the scientific study of heredity and hereditary variation

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Inheritance of Genes

• Each gene in an organism’s DNA has a specific locus on a certain chromosome

• We inherit one set of chromosomes from our mother and one set from our father

• Two parents give rise to offspring that have unique combinations of genes inherited from the two parents - sexual reproduction

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Asexual Reproduction

• In asexual reproduction, one parent produces genetically identical offspring by mitosis

Figure 13.2

Parent

Bud

0.5 mm

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Sexual Reproduction - The Human Life Cycle

• A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism

• Fertilization and meiosis alternate in sexual life cycles

• At sexual maturity the ovaries and testes produce haploid gametes by meiosis

• Unlike somatic cells, sperm and egg cells are haploid cells, containing only one set of chromosomes

• During fertilization, sperm and ovum fuse forming a diploid zygote

• The zygote develops into an adult organism

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

SpermCell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

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Meiosis• Reduces the chromosome number such that

each daughter• Cell has a haploid set of chromosomes• Ensures that the next generation will have:

– Diploid number of chromosome– Exchange of genetic information

(combination of traits– that differs from that of either parent)

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Meiosis• Only diploid cells can divide by meiosis.• Prior to meiosis I, DNA replication occurs.• During meiosis, there will be two nuclear divisions, and the result will be

four haploid nuclei.• No replication of DNA occurs between meiosis I and meiosis II.

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Meiosis

• Meiosis reduces the number of chromosome sets from diploid to haploid

• Meiosis takes place in two sets of divisions

– Meiosis I reduces the number of chromosomes from diploid to haploid

– Meiosis II produces four haploid daughter cells

Figure 13.7

Interphase

Homologous pairof chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicatedchromosomes

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2

Homologous chromosomes separate

Haploid cells withreplicated chromosomes

Sister chromatids separate

Haploid cells with unreplicated chromosomes

Meiosis I

Meiosis II

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Meiosis Phases

• Meiosis involves the same four phases seen in mitosis

prophase metaphase anaphase telophase

• They are repeated during both meiosis I and meiosis II.

• The period of time between meiosis I and meiosis II is called interkinesis.

• No replication of DNA occurs during interkinesis because the DNA is already duplicated.

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Prophase I• Prophase I occupies more than 90% of the time required for meiosis• Chromosomes begin to condense• In synapsis, the 2 members of each homologous pair of chromosomes

line up side-by-side, aligned gene by gene, to form a tetrad consisting of 4 chromatids

• During synapsis, sometimes there is an exchange of homologous parts between non-sister chromatids. This exchange is called crossing over

• Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred

Prophase Iof meiosis

Tetrad

Nonsisterchromatids

Chiasma,site of crossingover

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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

Sisterchromatids

Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Microtubuleattached tokinetochore

Tetrad

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

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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

Sisterchromatids

Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Microtubuleattached tokinetochore

Tetrad

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

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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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Prophase II• Meiosis II is very similar to mitosis• In prophase II, a spindle apparatus forms• In late prophase II, chromosomes (each still composed of

two chromatids) move toward the metaphase plate

Cleavagefurrow

PROPHASE II METAPHASE II ANAPHASE IITELOPHASE I ANDCYTOKINESIS

TELOPHASE II ANDCYTOKINESIS

Sister chromatidsseparate

Haploid daughter cellsforming

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Metaphase II• At metaphase II, the sister chromatids are 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

Cleavagefurrow





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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

Cleavagefurrow





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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

Cleavagefurrow





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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

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• Three events are unique to meiosis, and all three occur in meiosis l:

– Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information

– At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

– At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell. In anaphase II of meiosis, the sister chromatids separate

A Comparison of Mitosis and Meiosis

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MITOSIS MEIOSIS

Prophase

Duplicated chromosome(two sister chromatids)

Chromosomereplication

Chromosomereplication

Parent cell(before chromosome replication)

Chiasma (site ofcrossing over)

MEIOSIS I

Prophase I

Tetrad formed bysynapsis of homologouschromosomes

Metaphase

Chromosomespositioned at themetaphase plate

Tetradspositioned at themetaphase plate

Metaphase I

Anaphase ITelophase I

Haploidn = 3

MEIOSIS II

Daughtercells of

meiosis I

Homologuesseparateduringanaphase I;sisterchromatidsremain together

Daughter cells of meiosis II

n n n n

Sister chromatids separate during anaphase II

AnaphaseTelophase

Sister chromatidsseparate duringanaphase

2n 2nDaughter cells

of mitosis

2n = 6

A Comparison Of Mitosis And Meiosis

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Comparison

• Meiosis• DNA duplication

followed by 2 cell divisions

• Sysnapsis• Crossing-over• One diploid cell

produces 4 haploid cells

• Each new cell has a unique combination of genes

• Mitosis • Homologous

chromosomes do not pair up

• No genetic exchange between homologous chromosomes

• One diploid cell produces 2 diploid cells or one haploid cell produces 2 haploid cells

• New cells are genetically identical to original cell (except for mutation)

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Sexual Reproduction - The Human Life Cycle

• During fertilization, sperm and ovum fuse forming a diploid zygote

• The zygote develops into an adult organism

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

SpermCell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

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Spermatocytes to Spermatids

• Primary spermatocytes undergo meiosis I, forming two haploid cells called secondary spermatocytes

• Secondary spermatocytes undergo meiosis II and their daughter cells are called spermatids

• Spermatids are small round cells seen close to the lumen of the tubule

• Late in spermatogenesis, spermatids are nonmotile• Spermiogenesis – spermatids lose excess

cytoplasm and form a tail, becoming motile sperm

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Spermatogenesis

Figure 27.8b, c

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Oogenesis• Production of female sex cells by meiosis• In the fetal period, oogonia (2n ovarian stem cells) multiply

by mitosis and store nutrients• Primordial follicles appear as oogonia are transformed into

primary oocytes• Primary oocytes begin meiosis but stall in prophase I• From puberty, each month one activated primary oocyte

completes meiosis one to produce two haploid cells – The first polar body– The secondary oocyte

• The secondary oocyte arrests in metaphase II and is ovulated

• If penetrated by sperm the second oocyte completes meiosis II, yielding:

– One large ovum (the functional gamete)– A tiny second polar body

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Oogenesis

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