Introduction to Genetics: Meiosis Ch. 11 Ms. Haut

Introduction to Genetics:Meiosis

Ch. 11Ms. Haut

Introduction to Heredity

• Each organism must inherit a single copy of every gene from each of its “parents.”

• Offspring acquire genes from parents by inheriting chromosomes

• Inheritance is possible because:– Sperm and ova carrying each parent’s genes are

combined in the nucleus of the fertilized egg

Actual passing on of genes depends on the behavior of chromosomes

• Chromosomes -organizational unit of hereditary material in the nucleus of eukaryotic organisms

– Contain hundreds of thousands of genes, each of which is a specific region of the DNA molecule, or locus

Copyright Pearson Prentice Hall

Chromosome Number• Chromosome Number– All organisms have

different numbers of chromosomes.

– A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent.


Chromosome Number

• These two sets of chromosomes are homologous.

• Each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent.

• Chromosomes are the same size and shape and have the same genes in the same locations


Chromosome Number

• A cell that contains both sets of homologous chromosomes is said to be diploid.

• The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.

• For Drosophila, the diploid number is 8, which can be written as 2N=8.


Chromosome Number

• The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes.

• These cells are haploid. Haploid cells are represented by the symbol N.

• For Drosophila, the haploid number is 4, which can be written as N=4.

Human Life Cycle

• Each human somatic cell (body cell) has 46 chromosomes or 23 matching pairs (diploid)

Karyotype: male

Autosomes: non-sex chromosomes

Sex chromosomes:determine gender (XX; XY)

Human Life Cycle

• Gametes (sex cells) have a single set of 22 autosomes and a single sex chromosome, either X or Y

• With 23 chromosomes, they are haploid

haploid number: n = 23

diploid number: 2n = 46

Haploid sperm + haploid ova zygote (2n)

fertilization 2nn n

Fertilization

• Sperm compete to fertilize the egg

• Only one will win• Once sperm penetrates

the egg, egg creates a barrier to prevent other sperm from entering

• Only sperm nucleus is released inside egg

• Union of nuclei form zygote (fertilized egg)

Chromosomal Inheritance

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Phases of Meiosis• Phases of Meiosis– Meiosis is a process of reduction division in which

the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

– Meiosis involves two divisions, meiosis I and meiosis II.

– By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.


Phases of Meiosis

– Meiosis I

Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Interphase IMeiosis I


Phases of Meiosis• Cells undergo a round

of DNA replication, forming duplicate chromosomes.

Interphase I


Phases of Meiosis• Each chromosome

pairs with its corresponding homologous chromosome to form a tetrad.

• There are 4 chromatids in a tetrad.

MEIOSIS I Prophase I


Phases of Meiosis• When homologous chromosomes form tetrads in

meiosis I, they exchange portions of their chromatids in a process called crossing over.

• Crossing-over produces new combinations of alleles.


Phases of Meiosis• Spindle fibers attach

to the chromosomes. MEIOSIS I Metaphase I


Phases of Meiosis MEIOSIS I Anaphase I

• The fibers pull the homologous chromosomes toward opposite ends of the cell.


Phases of Meiosis MEIOSIS I Telophase I and Cytokinesis

• Nuclear membranes form. • The cell separates into two

cells.• The two cells produced by

meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.


Phases of Meiosis

• Meiosis II– The two cells produced by meiosis I now enter a

second meiotic division.– Unlike meiosis I, neither cell goes through

chromosome replication.– Each of the cell’s chromosomes has 2 chromatids.


Phases of Meiosis

• Meiosis II

Telophase II and Cytokinesis

Prophase II Metaphase II Anaphase IITelophase I and Cytokinesis I

Meiosis II


Phases of Meiosis• Meiosis I results in

two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.

MEIOSIS IIProphase II


Phases of Meiosis• The chromosomes

line up in the center of cell.

MEIOSIS II Metaphase II


Phases of Meiosis• The sister chromatids

separate and move toward opposite ends of the cell.

MEIOSIS II Anaphase II


Phases of Meiosis• Meiosis II results in four

haploid (N) daughter cells.

MEIOSIS II Telophase II and Cytokinesis


Gametogenesis

• Spermatogenesis– In male animals, meiosis

results in four equal-sized gametes called sperm.

– Sperm cells made in testes (over 100 million each day in adult male)

– Takes over 3 weeks to complete


Gametogenesis

• Oogenesis– In many female animals, only

one egg results from meiosis. The other three cells, called polar bodies

– Egg cells made in ovaries– Meiosis I takes place and

process stops before birth (1-2 million cells at this stage; by puberty only 400,000 left)

– Meiosis II begins each month once egg cell is released from ovary

Formation of Polar Body

• Occurs because of uneven division of cytoplasm during cytokinesis

• Egg cells must contain enough supplies to synthesize all the products needed by the developing embryo until the embryo implants into the uterine wall.

• Implantation does not occur until 7-10 days after fertilization.

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Comparing Mitosis and Meiosis

• Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.



• Mitosis– Cells produced by mitosis have the same number

of chromosomes and alleles as the original cell.– Mitosis allows an organism to grow and replace

cells.– Some organisms reproduce asexually by mitosis.



• Meiosis– Cells produced by meiosis have half the number of

chromosomes as the parent cell.– These cells are genetically different from the

diploid cell and from each other.– Meiosis is how sexually-reproducing organisms

produce gametes.

Meiosis


If the body cells of humans contain 46 chromosomes, a single sperm cell should have

a) 46 chromosomes.b) 23 chromosomes.c) 92 chromosomes.d) between 23 and 46 chromosomes.


During meiosis, the number of chromosomes per cell is cut in half through the separation of

a) daughter cells.b) homologous chromosomes.c) gametes.d) chromatids.


The formation of a tetrad occurs during a) anaphase I.b) metaphase II.c) prophase I.d) prophase II.


Compared to egg cells formed during meiosis, daughter cells formed during mitosis are

a) genetically different, while eggs are genetically identical.

b) genetically different, just as egg cells are.c) genetically identical, just as egg cells are.d) genetically identical, while egg cells are genetically

different.


In many female animals, meiosis results in the production of

a) only 1 egg.b) 1 egg and 3 polar bodies.c) 4 eggs.d) 1 egg and 2 polar bodies.

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Introduction to Genetics: Meiosis Ch. 11 Ms. Haut