Chromosome number • Human cells - Diploid – 46 total chromosomes per cell • 46 - Diploid number

10.1 An Overview of Meiosis

• Humans cells - 23 pairs of homologous chromosomes – 23 - Haploid number – The number of different kinds of chromosomes

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An Overview of Meiosis • Human cells are considered diploid because each cell has two copies – Some organisms • Haploid • triploid • tetraploid

Overview of Meiosis • Meiosis – Process of a single diploid cell dividing to produce four haploid cells • Cells that contain a single set of chromosomes • For reproduction

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Overview of Meiosis

Meiosis Compared to Mitosis

• Gametes – Haploid cells produced through meiosis are – Female gametes are eggs – Male gametes are sperm.

homologous pairs

Homologous means the same in size and function

Mitosis

1. Both mitosis and meiosis are initiated in cells that are diploid or “2n,” meaning cells that contain paired sets of chromosomes. The members of each pair are homologous––t he same in size and function. Two pairs of homologous chromosomes are shown within the cells in both the mitosis and meiosis figures. In each homologous pair, one chromosome (in red) comes from the mother of the person whose cell is undergoing meiosis, while the other chromosome (in blue) comes from the father of this person. duplication

Meiosis

somatic cell

gamete precursor 2n

2n

duplication

• They are the reproductive cells of human beings and many other organisms.

2n

2. Prior to the initiation of both mitosis and meiosis, the chromosomes duplicate. In both processes, each chromosome is now composed of two sister chromatids.

2n

2n

3. In mitosis, the chromosomes line up on the metaphase plate, one sister chromatid on each side of the plate. In meiosis, homologous chromosomes—not sister chromatids—line up on opposite sides of the metaphase plate.

2n

4. In mitosis, the sister chromatids separate. In meiosis, the homologous pairs of chromosomes separate. 2n

2n

division

division

2n

2n

1n

5. In mitosis, cell division takes place, and each of the sister chromatids from step 4 is now a full-fledged chromosome. Mitosis is finished. In meiosis, one member of each homologous pair has gone to one cell, the other member to the other cell. Because each of these cells now has only a single set of chromosomes, each is in the haploid or “1n” state. Next, these single chromosomes line up on the metaphase plate, with their sister chromatids on opposite sides of the plate.

1n

1n

1n

6. The sister chromatids of each chromosome then separate.

division

division 7. The cells divide again, yielding four haploid cells. 1n

1n

1n

1n

Figure 10.1

Overview of Meiosis • When the haploid sperm and haploid egg fuse, a diploid fertilized egg (or zygote) is produced, setting into development a new generation of organism.

10.2 The Steps in Meiosis

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The Steps in Meiosis • Meiosis – one round of chromosome duplication – followed by two rounds of cell division • No second chromosome duplication after first division

– meiosis I – meiosis II

1. Both mitosis and meiosis are initiated in cells that are diploid or “2n,” meaning cells that contain paired sets of chromosomes. The members of each pair are homologous––t he same in size and function. Two pairs of homologous chromosomes are shown within the cells in both the mitosis and meiosis figures. In each homologous pair, one chromosome (in red) comes from the mother of the person whose cell is undergoing duplication meiosis, while the other chromosome (in blue) comes from the father of this person.

Meiosis

somatic cell

• Two primary stages in meiosis

homologous pairs

Homologous means the same in size and function

Mitosis

The Steps in Meiosis

gamete precursor 2n

2n

duplication

2n

2. Prior to the initiation of both mitosis and meiosis, the chromosomes duplicate. In both processes, each chromosome is now composed of two sister chromatids.

2n

2n

3. In mitosis, the chromosomes line up on the metaphase plate, one sister chromatid on each side of the plate. In meiosis, homologous chromosomes—not sister chromatids—line up on opposite sides of the metaphase plate.

2n

2n

4. In mitosis, the sister chromatids separate. In meiosis, the homologous pairs of chromosomes separate.

2n

division

division

2n

2n

1n

1n

1n division

5. In mitosis, cell division takes place, and each of the sister chromatids from step 4 is now a full-fledged chromosome. Mitosis is finished. In meiosis, one member of each homologous pair has gone to one cell, the other member to the other cell. Because each of these cells now has only a single set of chromosomes, each is in the haploid or “1n” state. Next, these single chromosomes line up on sister chromatids of each then separate. the metaphase plate, with theirchromosome sister chromatids on 1n 6. The opposite sides of the plate.division 7. The cells divide again, yielding four haploid cells.

1n

1n

1n

1n

Meiosis I • Prophase I (after chromosome duplication) – First - pairing of homologous chromosomes – Crossing-over occurs • Homologous chromosomes exchange reciprocal sections of themselves • Increases variation • Results in no two sperm or eggs being identical

Meiosis I • Metaphase I – Homologous chromosome pairs line up at the metaphase plate

• One member of each homologous pair is on one side of the plate, the other member is on the other side – Random assortment

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Meiosis I • Anaphase I

Meiosis I • Telophase I

– Homologous pairs separate • each will become part of a separate daughter cell.

– separated Homologous pairs reach opposite poles

Meiosis I

Meiosis II

• Cytokinesis I – Two daughter cells fully separated • Now haploid • 23 chromosomes per cell – No homologous pairs present – Each chromosome still in duplicated state

• Meiosis II – Sister chromatids of the duplicated chromosomes are separated into separate daughter cells • No subsequent DNA replication • Proceeds much like mitosis from this point – Only 23 sets of sister chromatids present instead of 46

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Meiosis II

Meiosis II • Meiosis II

• Meiosis II

– Anaphase II • 23 sets of sister chromatids separate at centromere

– Prophase II

– Travel to poles

• Nuclear membranes breakdown

– Telophase II

– If they reformed at all after meiosis I – New mitotic spindle forms

• Separated chromosomes at the poles • Nuclear envelopes reform • Cleavage furrow begins to form

– Metaphase II

– Cytokenesis II

• 23 sister chromatids lined up on metaphase plate • Attached to mitotic spindle at the centromere

• Cleavage furrow grows to pinch off cell in to two new daughter cells – Now FOUR daughter haploid gametes, ready for maturation

Meiosis II (a) Meiosis I

Meiosis II Haploid

Diploid

cytokinesis

10.3 What is the Significance of Meiosis? cytokinesis End of interphase

Prophase I

DNA has already duplicated

Metaphase I

Homologous chromosomes link as they condense, forming tetrads.

Microtubules move homologous chromosomes to metaphase plate.

Crossing over occurs.

Independent assortment occurs.

Anaphase I Microtubules separate homologous chromosomes (sister chromatids remain together).

Telophase I

Prophase II

Two haploid daughter cells result from cytokinesis.

(Brief)

Metaphase II Sister chromatids line up at new metaphase plate.

Anaphase II

Telophase II

Sister chromatids separate.

Four haploid cells result.

Compare these cells to the cells above Telophase II

First important source of genetic variation

Second important source of genetic variation

(b) Crossing over

(c) Independent assortment

Exchange of parts of non-sister chromatids.

Random alignment of maternal/paternal chromosomes at the metaphase plate.

duplicated duplicated maternal paternal chromosome chromosome

sister chromatids

tetrad

Metaphase I

In the sequence above, homologous chromosomes lined up this way in Metaphase I . . .

Metaphase II

Metaphase I

... but they could have lined up this way, yielding a different outcome.

non-sister chromatids

Figure 10.2

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What is the Significance of Meiosis?

Meiosis Generates Diversity • Meiosis is unlike mitosis

• Meiosis – Generates diversity by ensuring that the gametes it gives rise to will differ genetically from one another.

– In mitosis, TWO daughter cells are exact genetic copies of parent cells • Diploid (46 chromosomes) • 2 copies of each homologous chromosome (23x2)

– In meiosis, FOUR daughter cells (gametes) are not identical • Haploid (23 chromosomes) • 1 copy of each chromosome

Meiosis Compared to Mitosis Homologous means the same in size and function

Mitosis

1. Both mitosis and meiosis are initiated in cells that are diploid or “2n,” meaning cells that contain paired sets of chromosomes. The members of each pair are homologous––t he same in size and function. Two pairs of homologous chromosomes are shown within the cells in both the mitosis and meiosis figures. In each homologous pair, one chromosome (in red) comes from the mother of the person whose cell is undergoing meiosis, while the other chromosome (in blue) comes from the father of this person. duplication

Meiosis

somatic cell

Meiosis

homologous pairs

gamete precursor 2n

2n

duplication

2n

– Crossing over – Independent assortment

2. Prior to the initiation of both mitosis and meiosis, the chromosomes duplicate. In both processes, each chromosome is now composed of two sister chromatids.

2n

2n

• Meiosis provides variation in gametes in two ways

3. In mitosis, the chromosomes line up on the metaphase plate, one sister chromatid on each side of the plate. In meiosis, homologous chromosomes—not sister chromatids—line up on opposite sides of the metaphase plate.

2n

4. In mitosis, the sister chromatids separate. In meiosis, the homologous pairs of chromosomes separate. 2n

2n

division

division

2n

2n

1n

5. In mitosis, cell division takes place, and each of the sister chromatids from step 4 is now a full-fledged chromosome. Mitosis is finished. In meiosis, one member of each homologous pair has gone to one cell, the other member to the other cell. Because each of these cells now has only a single set of chromosomes, each is in the haploid or “1n” state. Next, these single chromosomes line up on the metaphase plate, with their sister chromatids on opposite sides of the plate.

1n

1n

1n

division

6. The sister chromatids of each chromosome then separate. division 7. The cells divide again, yielding four haploid cells.

1n

1n

1n

1n

Figure 10.1

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Meiosis Generates Diversity

Meiosis Generates Diversity

• Crossing over – Prophase I of meiosis • Homologous chromosomes pair with each other • Chromosomes exchange reciprocal segments with one another • Tetrads – Aligned replicated homologous pairs

• Chiasma – Point on the chromosomes where crossing over occurs

Meiosis Generates Diversity

Meiosis Generates Diversity

• Independent assortment – Metaphase I of meiosis – Random alignment of maternal and paternal chromosomes (homologous pairs) on either side of the metaphase plate – Random chance alignment determines which daughter cell each chromosome (maternal or paternal) will end up in

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Meiosis Generates Diversity • Genetic diversity from meiosis and sexual reproduction – Largely responsible for the great diversity of life-forms seen in the living world today.

Meiosis Generates Diversity • Variation provided by meiosis and sexual reproduction – One of the major sources of variation for evolution to work upon – Through natural selection

• Asexual reproduction – In bacteria and other organisms • Offspring are exact genetic copies, or clones, of the parental organism • Variation only arises through mutation

Meiosis and Sex Outcome • Human females – 23 matched pairs of chromosomes

10.4 Meiosis and Sex Outcome

• 22 pairs of autosomes • one pair of sex-determining chromosomes – females are XX

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Meiosis and Sex Outcome

The X and the Y

• Human males – 22 autosomes – One pair of sex chromosomes • one X and one Y

Figure 10.4

Meiosis and Sex Outcome • Gametes – Each female egg contains • One X chromosome

– Each male sperm contains • One X, or • One Y

Meiosis and Sex Outcome • Male parent dictates gender of offspring – Egg fertilized by… • Sperm with a Y chromosome – Offspring will be male

• Sperm with an X chromosome – Offspring will be female

• Egg with X + sperm with X = XX = female • Egg with X + sperm with Y = XY = male

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10.5 Gamete Formation in Humans

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Gametogenesis

Gametogenesis • Spermatogenesis (continued)

• Spermatogenesis

– Male germ cell formation

– Male germ cell formation

spermatogonium

• Secondary spermatocytes

• Spermatogonia - Diploid - become: – More spermatogonia – Primary spermatocytes » Undergo meiosis I » Become secondary spermatocytes

Spermatogenesis

– Now TWO haploid cells – Undergo meiosis II – Become spermatids » Now FOUR haploid cells » Develop into spermatozoa

1. The diploid spermatogonium cell produces a primary spermatocyte.

Spermatogenesis

spermatogonium

primary spermatocyte

2. The primary spermatocyte goes through meiosis I, yielding two haploid secondary spermatocytes.

1. The diploid spermatogonium cell produces a primary spermatocyte.

Meiosis I

secondary spermatocytes primary spermatocyte 3. The secondary spermatocytes go through meiosis II, yielding four haploid spermatids, which will develop into mature sperm cells.

2. The primary spermatocyte goes through meiosis I, yielding two haploid secondary spermatocytes.

Meiosis II

Meiosis I spermatids

secondary spermatocytes

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Egg Formation

Egg Formation

• Oogenesis

• Oogenesis (continued)

– Female germ cell formation – Oogonium

Oogenesis

– One secondary oocyte

oogonium

• Will not complete meiosis II

• No longer produced after seven months gestation • Develops into primary oocyte Oogenesis

– Undergoes meiosis I – Now TWO haploid cells – Only ONE becomes secondary oocyte » Second cell becomes polar body » Due to unequal division of cell

oogonium

– Until fertilized by sperm – Upon fertilization meiosis II is completed – Results in: » ONE egg » THREE polar bodies Meiosis I

primary oocyte

Meiosis I

polar body

1. Before the birth of the female, a cell called an oogonium develops into a primary oocyte; this cell enters meiosis I, but remains there until it matures in the female ovary, beginning at puberty.

2. On average, one primary oocyte per month will complete meiosis I. In this process, an unequal meiotic division of cellular material leads to secondary the production of one oocyte polar body and one secondary oocyte, which enters into meiosis II.

Egg Formation • The vast majority of primary oocytes never complete meiosis, however. • It is only the single primary oocyte released each month, in the process of ovulation, that completes meiosis I.

polar body

primary oocyte

1. Before the birth of the female, a cell called an oogonium develops into a primary oocyte; this cell enters meiosis I, but remains there until it matures in the female ovary, beginning at puberty.

2. On average, one primary oocyte per month will complete meiosis I. In this process, an unequal meiotic division of cellular material leads to the production of one secondary polar body and one oocyte secondary oocyte, which enters into meiosis II. 3. Only secondary oocytes that are fertilized by sperm will complete meiosis II and develop into an egg. The three polar bodies that are produced by meiosis I and II will be degraded.

Meiosis II

polar bodies (will be degraded)

egg

Egg Formation • Only those ovulated oocytes that are fertilized by sperm complete meiosis II. • Only one of the cells produced in meiosis will have the potential to develop into a haploid egg.

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Sperm and Egg Formation Spermatogenesis

Oogenesis

oogonium

spermatogonium

1. The diploid spermatogonium cell produces a primary spermatocyte.

primary oocyte

primary spermatocyte

2. The primary spermatocyte goes through meiosis I, yielding two haploid secondary spermatocytes.

Meiosis I

secondary spermatocytes

polar body

3. The secondary spermatocytes go through meiosis II, yielding four haploid spermatids, which will develop into mature sperm cells.

1. Before the birth of the female, a cell called an oogonium develops into a primary oocyte; this cell enters meiosis I, but remains there until it matures in the female ovary, beginning at puberty.

10.6 Life Cycles: Humans and Other Organisms

2. On average, one primary oocyte per month will complete meiosis I. In this process, an unequal meiotic division of cellular material leads to secondary the production of one oocyte polar body and one secondary oocyte, which enters into meiosis II. 3. Only secondary oocytes that are fertilized by sperm will complete meiosis II and develop into an egg. The three polar bodies that are produced by meiosis I and II will be degraded.

Meiosis II

spermatids polar bodies (will be degraded)

egg

Figure 10.6

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Reproduction

Asexual Reproduction • Binary fission

• Sexual reproduction – Fusion of sperm and egg – Not all reproduction is sexual

– Bacteria • replicates its single chromosome and then divides in two

• Asexual reproduction – can take several forms: • binary fission • vegetative reproduction • regeneration

– Most types of organisms are capable of asexual reproduction • rare among more complex organisms • never carried out by mammals or birds.

cell wall

two daughter cells

chromosome cell membrane

parental bacterial cell

1.Bacterial cell starts with a single, circular chromosome attached to its plasma membrane.

2.The chromosome replicates and the daughter chromosomes attach to different sites on the plasma membrane.

3.The cell membrane and wall grow an extension between the attachment points of the two chromosomes.

4.The cell wall and membrane join together in the middle, resulting in two new cells.

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Asexual Reproduction

Asexual Reproduction

• Vegetative reproduction – Whole new plants grow from pieces of parent plant – Plants can do this in addition to sexual reproduction

Regeneration

• Regeneration – Worms and sea stars – A new, complete organism can be formed from a portion of an existing one – Similar to vegetative reproduction in plants

Gametogenesis Summary

PLAY

Figure 10.10

Figure 10.10

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