UNIT 3: MEIOSISREDUCTION DIVISION

Campbell & Reece, 2010 Chapter 13 by Dudrah Moyo

In humans, somatic cells (body cells) have:• 23 pairs of homologous chromosomes

and• one member of each pair from each parent.

The human sex chromosomes (Gonosomes)

X and Y differ in size and genetic composition. The other 22 pairs of chromosomes are

autosomes with the same size and genetic composition.

1. CHROMOSOMES ARE MATCHED IN HOMOLOGOUS PAIRS

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Homologous chromosomes are matched in:• length,• centromere position, and• gene locations (locus).

A locus (plural, loci) is the position of a gene.

Different versions (alleles) of a gene may be found at the same locus on maternal and paternal chromosomes.

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Centromere

Homologous chromosome pair

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Centromere

Humans and most animals and plants have diploid body cells.

That means they have two sets of chromosomes

(homologous chromosome pair) one from each parent. Diploid is written 2n. It refers to the total number of

chromosomes a cell can have.

2. GAMETES HAVE A SINGLE SET OF CHROMOSOMES

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Meiosis is a process that converts diploid nuclei to haploid nuclei.• Diploid cells have 2 sets of

chromosomes.• Haploid cells have 1 set of

chromosomes.• Meiosis occurs in the sex organs,

producing gametes—sperm and eggs. Fertilization is the fusion of a sperm and

egg cell. The zygote has a diploid chromosome

number, one set from each parent.

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Haploid gametes (n 23)

Egg cell

Sperm cell

Fertilization

n

n

Meiosis

Ovary Testis

Diploidzygote(2n 46)

2n

MitosisKey

Haploid stage (n)Diploid stage (2n)

Multicellular diploidadults (2n 46)

A life cycle

DIPLOID ZYGOTE

All sexual life cycles include an alternation between• a diploid stage and• a haploid stage.

Why is meiosis so important? It produces haploid gametes which prevents the chromosome number from doubling in every generation. Produce gametes for fertilization.

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3

Meiosis is a type of cell division that produces haploid gametes from diploid cells.

Two haploid gametes combine in fertilization to restore the diploid state in the zygote.

3. MEIOSIS

3

SUMMERY OF THE MEIOSIS PROCESS

MEIOSIS HAS 2 STAGES:

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MeiosIs I

INTERPHASE

PROPHASE I

METAPHASE I

ANAPHASE I

TELOPHASE I

Meiosis IIPROPHASE II

METAPHASE II

ANAPHASE II

TELOPHASE II

Cell build up energyDNA Replication (to

make duplicated chromosomes

Cell doesn’t change structurally.

MEIOSIS I : INTERPHASE

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Events occurring in the nucleus:• Chromosomes coil and become individual

chromosomes, nucleolus and nuclear envelope disappear.

• Homologous chromosomes come together as pairs by synapsis forming a tetrad (Each pair, with four chromatids)

• Non-sister chromatids exchange genetic material through the process of crossing over to ensure genetic variation.

• Centrioli move to opposite poles with spindle fibers between them.

MEIOSIS I : PROPHASE I

MEIOSIS I : PROPHASE I

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Genetic recombination is the production of new combinations of genes due to crossing over. Crossing over is an exchange of genes

between separate (non-sister) chromatids on homologous chromosomes.• Non-sister chromatids join at a chiasma

(plural, chiasmata), the site of attachment.• Genetic material are exchanged between

maternal and paternal (non-sister) chromatids.

CROSSING OVER

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CROSSING OVER

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Centrioli has reached the poles.

Homologous pairs align at the cell equator.

The two chromosomes attach to one spindle fiber by means of the kinetochore of the centromere.

.

MEIOSIS I: METAPHASE I

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Spindle fibers contract.

Duplicated chromosomes move to opposite poles.

.

MEIOSIS I: ANAPHASE I

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• Duplicated chromosomes have reached the poles.

• A nuclear envelope and nucleolus re-forms around chromosomes.

• Each nucleus now has the haploid number of chromosomes.

• Cell invaginates forming a cleavage furrow, which extends to for 2 separate haploid cells.

MEIOSIS I: TELOPHASE I

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Follows meiosis I without chromosome duplication.

Each of the two haploid products enters meiosis II.

MEIOSIS II

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• Chromosomes coil and become compact (if uncoiled after telophase I).

• Nuclear envelope and nucleolus, if re-formed, dissappears again.

• Centrioli move to opposite poles, forming spindle fibers between them.

MEIOSIS II: PROPHASE II

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• Individual duplicated chromosomes align on the equator.

• One chromosome per spindle fiber attached by means of kinetochore of centromere.

• Centrioli has reached the poles.

MEIOSIS II: METAPHASE II

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• Spindle fibers contract.• Duplicated

chromosomes split in half (centromere dividing in 2)

• Daughter chromosomes move to opposite poles.

MEIOSIS II: ANAPHASE II

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• Daughter chromosomes has reached the poles.

• Two cells invaginate and form 4 daughter haploid cells (gametes)

• They uncoil and form chromatin.

• Nuclear envelope and nucleolus for around chromatin again.

• Centrioli for centrosome.

MEIOSIS II: TELOPHASE II

SUMMERY OF MEIOSIS II

Prophase II Metaphase II Anaphase II

Haploid daughtercells forming

Telophase IIand Cytokinesis

Mitosis and meiosis both• begin with diploid parent cells that • have chromosomes duplicated during

the previous interphase. However the end products differ.

• Mitosis produces two genetically identical diploid somatic daughter cells.

• Meiosis produces four genetically unique haploid gametes.

4. SIMILARITIES AND DIFFERENCES BETWEEN

MITOSIS AND MEIOSIS

• Independent orientation at metaphase I

• Random fertilization.

• Crossing over of genes during prophase I

5. GENETIC VARIATION IN GAMETES RESULTS FROM:

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6. KARYOTYPE

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• A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs.

• Karyotypes allow for the observation of :

homologous chromosome pairs,

chromosome number, and chromosome structure.

SCIENTIST OBSERVING A HUMAN KARYOTYPE

Sex chromoso

KARYOTYPE

An extra copy of chromosome 21 causes Down syndrome or also known as TRISOMY 21.

A. Trisomy 21• involves the inheritance of three

copies of chromosome 21 and• is the most common human

chromosome abnormality.

7. ALTERATION IN CHROMOSOME NUMBER

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Down syndrome Karyotype

Trisomy 21 produces a characteristic set of symptoms, which include: • mental retardation,• characteristic facial features,• short stature,• heart defects,• susceptibility to respiratory infections,

leukemia, and Alzheimer’s disease, and• shortened life span.

The incidence increases with the age of the mother.

Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during:• meiosis I, if both members of a homologous

pair go to one pole or• meiosis II if both sister chromatids go to

one pole. Fertilization after nondisjunction yields

zygotes with altered numbers of chromosomes.

B. ACCIDENTS DURING MEIOSIS CAN ALTER CHROMOSOME

NUMBER

Nondisjunction

MEIOSIS I

MEIOSIS II

Normalmeiosis II

Gametes

Number ofchromosomes

Abnormal gametes

n 1 n 1 n 1 n 1

Normalmeiosis I

MEIOSIS I

MEIOSIS II

Nondisjunction

Abnormal gametes Normal gametes

n 1 n 1 n n

Sex chromosome abnormalities tend to be less severe, perhaps because of• the small size of the Y

chromosome or• X-chromosome inactivation.

C. ABNORMAL NUMBERS OF SEX CHROMOSOMES

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In general,• a single Y chromosome is enough to

produce “maleness,” even in combination with several X chromosomes, and

• the absence of a Y chromosome yields “femaleness.”

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The following table lists the most common human sex chromosome abnormalities.

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Errors in mitosis or meiosis may produce polyploid species, with more than two chromosome sets.

.

D. NEW SPECIES CAN ARISE FROM ERRORS IN CELL DIVISION

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Chromosome breakage can lead to rearrangements that can produce:

• genetic disorders or,

• if changes occur in somatic cells, cancer.

8. ALTERATIONS OF CHROMOSOME STRUCTURE

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• a deletion, the loss of a chromosome segment,

• a duplication, the repeat of a chromosome segment,

• an inversion, the reversal of a chromosome segment, or

• a translocation, the attachment of a segment to a non-homologous chromosome that can be reciprocal.

THESE REARRANGEMENTS MAY INCLUDE:

THESE REARRANGEMENTS MAY INCLUDE:

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Deletion

Duplication

Inversion

Reciprocal translocation

Homologouschromosomes Nonhomologous

chromosomes

LIST OF SOURCES USED

• http://www.slideshare.net/freesmart/meiosis-powepo?qid=fe02f4ec-0e9e-4a97-a258-1795f193ff7e&v=qf1&b=&from_search=1

• http://www.slideshare.net/CDA-PamelaOrtiz/meiosis-ppt-14835989

• Mrs J.Williamsons 2014 sildes 3A FET MEIOSIS

• http://www.slideshare.net/RawanShahien/mitosis-and-meiosis-6360090