Meiosis and Sexual Reproduction

n

n 2n

Review Question

Discussion Question

Mitosis ReviewSingle-cell organisms (prokaryotic) carry

out Mitosis for asexual reproductionMulti-cell organisms (eukaryotics) carry

out Mitosis for growth and repairThe phases of mitosis in order are:

Prophase, Metaphase, Anaphase, and Telophase

Interphase is known as the “resting phase”

Cytokinesis occurs at the same time as telophase

Through which process do eukaryotic cells reproduce?

Requires sex cells from two parents (male and female)

Sex cells form as a result of meiosisOffspring are usually not identicalImportant for the survival of many

species◦Process results in genetic variation of

offspring

Sexual Reproduction

Define the following terms:

Sexual Reproduction

MeiosisGameteSomatic CellsHaploid

Textbook pg 46 - 51

• Fertilization• Zygote• Diploid• Homologous

Pairs

New Vocabulary

Meiosis: The process that results in the formation of gametes

Gametes: Sex cells (egg in females; sperm in males); contains haploid chromosomes

Somatic Cells: All body cells except for sex cells; contain Diploid chromosomes

Haploid: another term for half; refers to the number of chromosomes; written as n◦Chromosomes found singular

Fertilization: the union of an egg and a sperm

Zygote: Cell that forms as a result of fertilization◦Contains Diploid chromosomes (2n)

Diploid: Chromosomes found in pairs; written as 2n; Somatic Cells

Homologous Chromosomes: Similar chromosomes that are found in pairs; ½ of pair comes from mother, other ½ from father◦Human Body somatic cells contain 46

chromosomes, so 23 homologous pairs

MeiosisTwo division of the nucleus occursSteps of each division are similar to

those of mitosisMeiosis I

◦Each chromosome is duplicated, similar to mitosis

Meiosis II◦Two cells that form as a result of Meiosis I

divide again◦Four daughter cells are produced

Video

Meiosis

Parent cell – chromosome pair

Chromosomes copied

1st division - pairs split

2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Meiosis IPhase Image Description

Prophase I

- Homologous chromosomes pair together - Crossing over occurs between pairs resulting in genetic variation

Metaphase I

- Spindle fibers attach to chromosomes and line up in the middle of the cell

Anaphase I - Spindle fibers pull the homologous pairs apart- Chromosomes move to opposite sides of the cell

Telophase I

(& cytokinesi

s)

-Nuclear Membrane forms-Cell splits into two new cells- Cells contain haploid chromosomes (n)

Meiosis IIPhase Image Description

Prophase II

-new Spindle fibers form and attach to sister chromatids

Metaphase II

- Spindle fibers attach to sister chromatids and line up in the middle of the cell

Anaphase II

- Spindle fibers pull the sister chromatids apart- Chromosomes move to opposite sides of the cell

Telophase II(&

cytokinesis)

-Nuclear Membrane forms-Each cell splits into two new cells-Forms 4 haploid cells

What is the end result of Meiosis?

End Results of MeiosisFemale Eggs are MUCH larger than

male spermEach haploid cell in males become a

gamete◦Results in 4 sperm

Only 1 of the haploid cells in females become a gamete◦Results in 1 egg and 3 polar bodies◦Polar bodies are unused and broken down

K K K K

K K K K

EggPolar Bodies Why does

only one gamete form in Females?

How do Mitosis and Meiosis differ?

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in

Meiosis

Mitosis The first (and distinguishing) division of meiosis

Crossing-Over

Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

Explain the significant of crossing over.

Another Way Meiosis Makes Lots of Different Sex Cells

Another Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment

Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.

That’s a lot of diversity by this mechanism alone.

In humans, n = 23 and 223 = 6,000,0000.

Meiosis – division error

Chromosome pair

Describe the errors that occur during each division.

Meiosis error - fertilization

Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’.

In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

21 trisomy – Downs Syndrome

Can you see the extra 21st chromosome?