3-1 Notes - Sexual Reproduction & Meiosis Chapter 3, Lesson 1.

3-1 Notes - 3-1 Notes - Sexual Reproduction & Sexual Reproduction & MeiosisMeiosis

Chapter 3, Lesson 1Chapter 3, Lesson 1

Sexual ReproductionSexual Reproduction

• Reproduction in organisms produces new Reproduction in organisms produces new offspring.offspring.


• Organisms can reproduce their cells in two Organisms can reproduce their cells in two ways: by sexual reproduction and by ways: by sexual reproduction and by asexual reproduction. asexual reproduction.

Sexual Reproduction Asexual Reproductionneeds two organisms of the same species only needs one organism

child is different from the parents child is an exact copy of the parent

special cells called gametes are created no special cells involved

Sexual vs. Asexual Reproduction


• Sexual reproduction occurs when the DNA Sexual reproduction occurs when the DNA from from twotwo differentdifferent cells combine. cells combine.

23 chromosomes 23 chromosomes

46 chromosomes46 chromosomes

46 chromosomes


• Half the DNA is contained in an egg cell.Half the DNA is contained in an egg cell.

• Half the DNA is contained in a sperm cell.Half the DNA is contained in a sperm cell.



• In a process called fertilization, the In a process called fertilization, the sperm and egg cells fuse together sperm and egg cells fuse together forming something called a forming something called a zygotezygote..

Advantages of Sexual Advantages of Sexual ReproductionReproduction

• Genetic variation – a variety of genetic Genetic variation – a variety of genetic traits in a species makes it more likely for traits in a species makes it more likely for the species to survive changes in the species to survive changes in environmental conditions.environmental conditions.


• Selective breeding (the opposite) – when Selective breeding (the opposite) – when males and females with preferred traits are males and females with preferred traits are chosen to breed so they produce offspring chosen to breed so they produce offspring with the preferred traits – this with the preferred traits – this reducesreduces genetic variation (offspring are very genetic variation (offspring are very similar).similar).

Disadvantages of Sexual Disadvantages of Sexual ReproductionReproduction

• Getting egg and sperm together for Getting egg and sperm together for fertilization can be difficult.fertilization can be difficult.

Disadvantages of Sexual Disadvantages of Sexual ReproductionReproduction

• It takes time for organisms to grow and It takes time for organisms to grow and develop before they are mature enough to develop before they are mature enough to reproduce.reproduce.

Importance of MeiosisImportance of Meiosis

• Meiosis is a type of cell division that Meiosis is a type of cell division that produces sperm or egg cells.produces sperm or egg cells.


• Meiosis ensures that offspring inherit the Meiosis ensures that offspring inherit the correct number of chromosomes.correct number of chromosomes.


46 chromosomes46 chromosomes

46 chromosomes


• Without meiosis, the chromosome number Without meiosis, the chromosome number would double with each generation.would double with each generation.

Diploid vs. Haploid CellsDiploid vs. Haploid Cells

• DiploidDiploid cells have 2 sets cells have 2 sets of each homologous of each homologous chromosome (they are chromosome (they are similar, but not identical).similar, but not identical).

• HaploidHaploid cells have 1 set cells have 1 set from each homologous from each homologous chromosome pair.chromosome pair.


• A diploid human cell has 23 pairs of (or A diploid human cell has 23 pairs of (or 46 total) homologous chromosomes.46 total) homologous chromosomes.

MeiosisMeiosis

• Meiosis is split into two overall stages: Meiosis is split into two overall stages: Meiosis I and Meiosis II. Meiosis I and Meiosis II.

MeiosisMeiosis

1.1. Meiosis IMeiosis I• Prophase I – Nuclear membrane breaks apart Prophase I – Nuclear membrane breaks apart

and pairs of replicated chromosomes and pairs of replicated chromosomes condense. condense. (92 human chromosomes)(92 human chromosomes)

MeiosisMeiosis

1.1. Meiosis IMeiosis I• Metaphase I – Sister chromatids line up along Metaphase I – Sister chromatids line up along

the center of the cell. the center of the cell. (92 human (92 human chromosomes)chromosomes)

MeiosisMeiosis

1.1. Meiosis IMeiosis I• Anaphase I – Sister chromatids move to Anaphase I – Sister chromatids move to

opposite ends of the cell. opposite ends of the cell. (92 human (92 human chromosomes)chromosomes)

MeiosisMeiosis

1.1. Meiosis IMeiosis I• Telophase I – Nuclear membrane forms Telophase I – Nuclear membrane forms

around each set of sister chromatids and around each set of sister chromatids and the cytoplasm divides, forming two daughter the cytoplasm divides, forming two daughter cells. cells. (46 human chromosomes)(46 human chromosomes)

MeiosisMeiosis

2.2. Meiosis IIMeiosis II• Prophase II – Chromosomes do not replicate Prophase II – Chromosomes do not replicate

before nuclear membrane breaks apart. before nuclear membrane breaks apart. (46 (46 human chromosomes)human chromosomes)

MeiosisMeiosis

2.2. Meiosis IIMeiosis II• Metaphase II – Chromosomes line up along Metaphase II – Chromosomes line up along

the center of the cell. the center of the cell. (46 human (46 human chromosomes)chromosomes)

MeiosisMeiosis

2.2. Meiosis IIMeiosis II• Anaphase II – Sister chromatids separate, Anaphase II – Sister chromatids separate,

moving to opposite ends of the cell. moving to opposite ends of the cell. (46 (46 human chromosomes)human chromosomes)

MeiosisMeiosis

2.2. Meiosis IIMeiosis II• Telophase II – Nuclear membrane reforms Telophase II – Nuclear membrane reforms

and the cytoplasm divides. and the cytoplasm divides. (23 human (23 human chromosomes)chromosomes)

MeiosisMeiosis

• Meiosis results Meiosis results in 4 haploid in 4 haploid nuclei from 1 nuclei from 1 diploid nucleus.diploid nucleus.

MeiosisMeiosis

• Meiosis I Meiosis I produces 2 produces 2 genetically genetically different cells.different cells.

MeiosisMeiosis

• Meiosis II Meiosis II produces 2 produces 2 identical haploid identical haploid cells from each cells from each cell produced in cell produced in Meiosis I.Meiosis I.

What is the new cell that forms from fertilization called?

A sperm

B egg

C haploid

D zygote

3.1 Sexual Reproduction and Meiosis

How many chromosomes from each homologous pair does a haploid cell contain?

A one

B two

C three

D four


How many daughter cells are produced in meiosis?

A one

B two

C three

D four


What is one advantage of sexual reproduction?

A produces many offspring

B offspring are genetically identical

C offspring have more genetic variation

D can produce offspring quickly

How many times does division of the nucleus and cytokinesis happen in meiosis?

A one

B two

C three

D four

SCI 2.a

3-1 Notes - Sexual Reproduction & Meiosis Chapter 3, Lesson 1.

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