Meiosis

B-4.5 Summarize the characteristics of the phases of meiosis I and meiosis II.

Key Concepts

n  Daughter cells n  Diploid n  Haploid n  Zygote n  Gamete

n  Meiosis I vs. Meiosis II

What You Already Know…

This concept has not been addressed in previous science classes. It is all brand new.

It is Essential For You to Know…

n  The process of meiosis and its importance to sexual reproduction just as mitosis is to asexual reproduction.

n  Meiosis occurs in two steps: Meiosis I and Meiosis II

OBJECTIVES n  Summarize the phases of Meiosis I and

Meiosis II. n  Illustrate and interpret scientific

diagrams of the phases of meiosis. n  Compare meiosis I and meiosis II. n  Compare mitosis and meiosis with regard

to processes and outcomes. n  Explain the effect of crossing over on the

genetic variation in daughter cells.

Vocabulary

1.  Meiosis 2.  Gametogenesis 3.  Sperm 4.  Egg 5.  Polar body 6.  Zygote

Meiosis

Remember… n  Somatic cells reproduce by

mitosis.

n  Somatic cells are 2N. Gametes must be N.

n  The mother and father each contribute ½ of the offspring’s DNA or chromosomes.

n  Meiosis reduces the number of chromosomes by half.

Textbook Reading

Let’s read page 173 in your textbook.

Questions

1.  What’s the difference between homologous chromosomes & sister chromatids?

2.  What connects two sister chromatids?

3.  How are sister chromatids made?

4.  When are homologous chromosomes divided?

5.  When are sister chromosomes divided?

homologous chromosomes

sister chromatids

sister chromatids

–  Pairs of homologous chromosomes separate in meiosis I.

–  Homologous chromosomes are similar but not identical. –  Sister chromatids divide in meiosis II. –  Sister chromatids are copies of the same chromosome.

Why Meiosis?

n  Gametes (sex cells) don’t divide (primary spermatocyte or oocyte)

n  Diploid number in a species remains the same

(meiosis reduces 2n to n, then n+n=2n)

n  Crossing over (Prophase I) produces genetic variety.

Textbook Pages

Now, look at pages 174 – 175 in your textbook.

Meiosis

n  Haploid gametes (egg and sperm) are produced from diploid cells through a process called meiosis. n  2N à N n  A process of reduction division in which the number of

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

n  2 distinct phases: Meiosis I and Meiosis II

Meiosis I n  Interphase : Primary 2n oocyte/

spermatocyte replicates its chromosomes becoming 4n (tetrad=4 homologous chromosomes)

2n

4n

n  Meiosis I occurs after DNA has been replicated.

n  Meiosis I divides homologous chromosomes in four phases.

n  Prophase I: 2 homologous chromosomes move together and exchange various aligned genes...CROSSING OVER.

•  Crossing Over adds genetic variety—this is why full siblings are different.

n  Metaphase I: tetrad moves to middle and attaches to spindle.

n  Anaphase I: spindle contracts pulling pairs of homologous chromosomes to each pole.

n  Telophase I: 2 new cells are formed, each 2n. Pairs of chromosomes still attached at centromere.

n  Meiosis II divides sister chromatids in four phases.

n  DNA is not replicated between meiosis I and meiosis II.

n  Prophase II: Spindle and centrioles move to new poles of each cell.

n  Metaphase II: Sister Chromatids in each cell move to the middle and attach to the spindle.

n  Anaphase II: Spindle contract pulling sister chromatids apart.

n  Telophase II: Each cell divides forming 4 haploid daughter cells.

Textbook Pages

Read page 176 in your textbook.

Questions

1.  What is the end result of meiosis?

2.  What is gametogenesis?

3.  What is the female gamete? The male?

4.  How many sperm are formed?

5.  How many eggs are formed? What else is formed with the eggs?

Gamete Formation n  Spermatogenesis

n  4 sperm are produced after Meiosis

n  Oogenesis n  1 egg and 3 polar

bodies are produced after Meiosis

Gamete Formation n  Gametogenesis is the production

of gametes. n  Gametogenesis differs between

females and males.

n  Sperm become streamlined and motile.

n  Sperm primarily contribute DNA to an embryo.

n  Eggs contribute DNA, cytoplasm, and organelles to an embryo.

n  During meiosis, the egg gets most of the contents; the other cells form polar bodies.

Males vs. Females

n  Males are born with spermatocytes that turn into sperm once puberty happens.

n  Spermatogenesis will continue throughout a male’s lifetime.

n  Females are born with oocytes that turn into an egg once puberty occurs.

n  Oogenesis will occur only until a female hits menopause.

n  The egg must either be fertilized or shed through menstruation.

Sperm Cell 23 chromosomes

(haploid n)

Egg Cell 23 chromosomes

(haploid n)

Baby 46 chromosomes

(diploid 2n)

Why Meiosis?

Meiosis differs from mitosis in significant ways.

n  Meiosis has two cell divisions while mitosis has one. n  In mitosis, homologous chromosomes never pair up. n  Meiosis results in haploid cells; mitosis results in diploid

cells.