Cell ReproductionMeiosis

aka Cell Division

MeiosisCell division where one diploid cell (2n)

produces four haploid (n) cells called sex cells or gametes

These cells have half the number of chromosomes as a body cell of a parent

Meiosis vocabulary

What is a gamete?Male and female sex cells; the

sperm and eggWhat is the male gamete?Sperm; has 23 chromosomesWhat is the female gamete?Egg; has 23 chromosomes

Meiosis vocabulary…What is a zygote?A fertilized egg – has a diploid number

(2n) of chromosomesWhat is sexual reproduction?The pattern of reproduction that

involves the production and fusion of haploid sex cells

What is a diploid cell?Cell containing two of each kind of

chromosome from the parent

Meiosis vocabulary…What is a haploid cell?Cell containing one of each kind of

chromosome from the parentWhat is an allele?Gene form for each variation of a trait of

an organism. Example: gene for height can express tall or short

Homologous chromosomes:Pairs of like chromosomes even though the

alleles may be different – so they are not identical chromosomes – sibling variation

The Phases of Meiosis

Meiosis I

InterphaseProphase IMetaphase IAnaphase ITelophase I

Meiosis II

Prophase IIMetaphase IIAnaphase IITelophase II

Interphase

Carries out metabolic activities of the cell

Replicates its chromosomes

Precedes meiosis 1

Prophase I

• Chromosomes coil up and spindle form

• Homologous chromosomes pair up and a four-part structure called a tetrad forms

During prophase I…

• Tightly wound tetrads get so close to one another that the genetic material from one homologous pair exchanges with the other creating new combinations of alleles in the genetic code

• Nuclear membrane disappears• Nucleolus disappears

What is crossing over?Exchange of genetic material by non-sister chromatids during prophase I

Results in new combinations of alleles

Metaphase I

• Tetrads line up at the equator• Homologous chromosomes are lined

up in pairs along the midline or equator of the cell

• Spindle fibers attach to the centromere on each pair

• The pairs are not attached to each other

Anaphase I

• Homologous chromosomes are separated and pulled to opposite poles of the cell

• The centromeres do not split during this phase

• Each new cell receives only one chromosome from each homologous pair

Telophase I• Spindle breaks down• Chromosomes uncoil• Cytoplasm divided in 2• Each cell has half the genetic

information from the original cell• Chromosomes are still doubled

containing two sister chromatid• Nuclear membrane reforms• Two diploid daughter cells formed

Meiosis II

Is there an interphase in Meiosis II?No. Chromosomes do not replicateThe cells go directly from meiosis I to

meiosis II

Prophase II

• Spindle forms in each of the two new cells

• Chromosomes are still coiled as sister chromatids

• Nuclear membrane disappears

• Nucleolus disappears

Metaphase II

• Chromosomes line up at the equator of the cell

• Spindle fibers attach to the centromeres

Anaphase II

• Centromeres split

• Spindle fibers pull separated chromatids to opposite poles of the cell

Telophase II

• Spindle fibers disappear• Chromosomes uncoil• Nuclear membrane reforms• Nucleolus reforms• Cytoplasm divides• Each cell now has half the

chromosomes that its parents had• Four daughter haploid cells are

formed

What are the two divisions of meiosis?

Meiosis I and Meiosis II

Meiosis IWhen homologous chromosomes come

together and separateMeiosis IIThe division that results in 4 daughter

cells, each with half the chromosomes present in the original cell

What is genetic recombination?

Genetic variation due to crossing over and random assortment

How many ways can the chromosomes line up?

2

How do you find the total number of different kinds of sperm and egg that can be

produced?• Take the number of pairs and place it

as the exponent with a base of 2• Example: in humans there are 23

pairs of chromosomes• For sperm: 223 = over 8 billion• For egg: 223 = over 8 billion

What is the total when fertilization occurs?

223 * 223 = 70 trillion zygotes are possible

What is oogenesis?

Refers to the formation of the egg cell

What is spermatogenesis?

Refers to the formation of the sperm cell

Practice Quiz

Get out 1 sheet of blank paper. No half sheets of paper.

Practice quiz

1.How many cell are produced from the cell division of meiosis?

2. Does the DNA replicate itself before meiosis II?

3. What is the stage where the tetrad forms?

Practice quiz…4. What are the stages of meiosis? (in

order)

5. In what phase does the DNA double?

6. How many cells are produced from spermatogenesis?

7. How many cells are produced from oogenesis?