Meiosis grade 8

Unit 7 Lesson 2 Meiosis

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Indiana Standards


• 8.3.3 Explain that genetic information is transmitted from parents to offspring mostly by chromosomes.

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How do sex cells differ from body cells?• Before sexual reproduction can take place, each

parent produces sex cells.

• Sex cells have half of the genetic information that body cells have.

• When genetic information from two parents combines, the offspring will have one full set of genetic information.


How do sex cells differ from body cells?• In body cells, chromosomes are found in pairs of

homologous chromosomes, which have the same structure and size.

• Homologous chromosomes carry the same genes. However, some may be different versions of the genes.



How do sex cells differ from body cells?• One chromosome pair is made up of sex

chromosomes.

• Cells with homologous chromosomes are called diploid.



Why do organisms need sex cells?• Normal human body cells contain 46

chromosomes.

• Sex cells are also known as gametes. Gametes have only half of the usual number of chromosomes.

• Gametes are haploid, meaning they have one chromosome from each homologous pair. Gametes are made in the reproductive organs.



Why do organisms need sex cells?• Sex cells are needed so that in sexual

reproduction offspring receive a total of 46 chromosomes.



How are sex cells made?• Mitosis produces two new cells that contain exact

copies of the chromosomes of the parent cell.

• A different kind of cell division is needed to produce sex cells.

• Meiosis is the type of cell division that produces haploid sex cells such as sperm or egg cells.

• When an egg is fertilized by a sperm cell, a new diploid cell forms.



One Step at a Time


What are the stages of meiosis?• Meiosis has two parts: meiosis I and meiosis II.

• Recall that homologous chromosomes have the same genes but are not exact copies of each other.


One Step at a Time


What are the stages of meiosis?• Before meiosis I begins, each chromosome is

duplicated. Each half is called a chromatid.

• Chromatids are connected by centromeres.

• During meiosis I, pairs of homologous chromosomes and sex chromosomes split apart into two new cells.


What are the stages of meiosis?• In prophase I, duplicated homologous

chromosomes pair up.



What are the stages of meiosis?• In metaphase I, the homologous chromosome

pairs line up in the middle of the cell.



What are the stages of meiosis?• In anaphase I, the homologous chromosomes

separate from their partners and move to opposite sides.



What are the stages of meiosis?• In telophase I and cytokinesis, the nuclear

membranes re-form and the cell divides into two cells. The chromatids are still joined.



What are the stages of meiosis?• Meiosis II involves both of the new cells that

formed during meiosis I.

• The new cells divide during meiosis II.

• Meiosis II results in four haploid sex cells.



What are the stages of meiosis?• In males, the four sex cells develop into sperm

cells.

• In females of some species, three cells are broken down, and only one haploid cell becomes an egg.



What are the stages of meiosis?• In prophase II, the nuclear membrane breaks

apart.



What are the stages of meiosis?• In metaphase II, the chromosomes line up in the

middle of the cells.



What are the stages of meiosis?• In anaphase II, the chromatids are pulled apart

and move to opposite sides.



.

What are the stages of meiosis?• In telophase II and cytokinesis, the nuclear

membranes re-form and the cells divide. Each cell is haploid.



How does meiosis compare to mitosis?• Only sex cells undergo meiosis. All other cells

divide by mitosis.

• In meiosis, chromosomes are copied once, and the nucleus divides twice.

• In mitosis, the chromosomes are copied once, and the nucleus divides once.



How does meiosis compare to mitosis?• Meiosis produces haploid cells. Mitosis produces

diploid cells.




Down Syndrome


• Down syndrome is a genetic disease caused by an error during meiosis.

• The chromatids in chromosome 21 do not separate, so one of the sex cells gets an extra copy of chromosome 21.

• Down syndrome causes a number of health problems and learning difficulties, but many people with Down syndrome have fulfilling lives.

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Meiosis grade 8