MEIOSIS DIRECTED READING

Write and answer for the following questions.• What is meiosis?• Explain the difference between meiosis I and meiosis II.• List the stages of meiosis in the order that they occur. • What is crossing-over?• What is independent assortment? During which phase(s) of meiosis does

independent assortment occur and what is the significance of this process?

For 6-13, Write the name of the stage of meiosis that is being described.6. The centromeres divide, and the chromatids move to opposite poles of

the cell.7. The homologous chromosomes separate. The chromosomes of each pair

are pulled to opposite poles of the cell by the spindle fibers. The chromatids do not separate at their centromeres.

8. The chromosomes condense, and the nuclear envelope breaks down. Homologous chromosomes pair all along their length and then crossing-over occurs.

9. After one division of the nucleus, a new spindle forms around each group of chromosomes.

10. Individual chromosomes line up along the equator, attached at their centromeres to spindle fibers.

11. A nuclear envelope forms around each set of chromosomes. Two cells undergo cytokinesis, forming haploid offspring cells.

12. Individual chromosomes gather at each of the two poles. In most organisms, the cytoplasm divides, forming two new cells.

13. The pairs of homologous chromosomes are moved by the spindle to the equator of the cell. The homologous chromosomes, each made up of two chromatids, remain together.

Steps of meiosis Worksheets: Should be kept in your notebooks. I will check these off.

Worth 50 pts = 2 homework assignments.• Use the worksheet to guide your work but don’t

write on it.1-8 Draw the chromosomes in the cells.

• This organism is 2n=4 (meaning that the organism has _?_ sets of chromosomes with _?_ chromosomes per set. _?_ set from _?_, _?_ set from _?_))

• Illustrate the major events in each stage’s cell by drawing the appropriate organelle and the correct position of the chromosomes/chromatids as well.• Be careful on metaphase I & metaphase II…

• To the right of the cell, label the stage & summarize all the major events (2-3 each) for each stage.• Leave enough room for extras.

9-17: Analyze the content.• You may work together to make sure you learn

the most.• You have the rest of today and tomorrow to

complete this assignment.Reference pgs. 250-254

WARM UP… ARE YOU TRACKING?

A

D

C

B

Diploid

Diploid

Diploid

Haploid

ESSENTIAL QUESTION

• How does an organism produce gametes that will combine with the gamete of a mate to form genetically diverse offspring?

FORMATION OF GAMETES

• Meiosis: cell division that produces gametes with half the number of chromosomes that are in the parent cell.

• Basically, it’s the process that takes germ cells (aka sex cells) that have 2 sets of chromosomes and forms gametes that have 1 set of chromosomes & are capable of fusing with other opposite sex gametes.

• Before you learn the steps to meiosis it’s important to discuss chromosome numbers first to see why it occurs.

READ THE STATEMENTS BELOW AND DO YOUR BEST TO FILL IN THE

BLANKS.

• The cell on the left is the starting ______ cell.• It is considered _______ because it has ____ sets of

chromosomes.• One set comes from _____ and one set comes from ____.• The cell on the right is the product of meiosis called a

__________. • It is considered ________ because it has ____ set of

chromosomes.

g e r m

d i p l o i d t w o

m o m d a d

g a m e t e

h a p l o i d o n e

OBJECTIVES• Discussion:• Compare the number of sets of chromosomes between a

haploid cell and a diploid cell.• Discovery: • Summarize the different phases of meiosis.

• Explain how the function of meiosis differs from the function of mitosis.

• Describe three mechanisms of genetic variation.

STAGES OF MEIOSIS: SUMMARIZED

• In summary, meiosis forms haploid gametes from diploid germ cells but it’s a little more complicated.

• During meiosis, the original diploid cell goes through two divisions to form four haploid cells.

• In the first stage, meiosis I, homologous chromosomes are separated.

• In the second stage, meiosis II, the sister chromatids of each homologue are separated.

STAGES OF MEIOSIS, CONTINUED

Meiosis I• Meiosis begins with a diploid cell that has copied

its chromosomes.

• During prophase I, 1. the chromosomes condense2. the nuclear envelope breaks down3. homologous chromosomes pair 4. chromatids exchange genetic material in a process

called crossing-over. • This is a very important step in the process and the

one that results in the genetic variation among siblings.

STAGES OF MEIOSIS, CONTINUED

Meiosis I• In metaphase I

1. the spindle moves the pairs of homologous chromosomes to the equator of the cell

2. the homologous chromosomes remain together

Homologous chromosomesForms a “Tetrad”

STAGES OF MEIOSIS, CONTINUED

Meiosis I• In anaphase I

1. the homologous chromosomes separate

2. the spindle fibers pull the chromosomes of each pair to opposite poles of the cell

3. the chromatids do not separate at their centromeres. Each chromosome is still made of two chromatids. The genetic material, however, has recombined.

STAGES OF MEIOSIS, CONTINUED

Meiosis I• During telophase I

1. the cytoplasm divides (cytokinesis)2. two new cells are formedBoth cells have one chromosome from

each pair of homologous chromosomes.

STAGES OF MEIOSIS, CONTINUEDMeiosis II• Meiosis II begins with the

two cells formed at the end of telophase I of meiosis I.

• Note: The chromosomes are not copied between meiosis I and meiosis II.

• In prophase II1. new spindles form.

• During metaphase II1. the chromosomes line up

along the equators and are attached at their centromeres to spindle fibers.

STAGES OF MEIOSIS, CONTINUED

Meiosis II• In anaphase II

1. the centromeres divide 2. the chromatids, which are now called chromosomes,

move to opposite poles of the cell.

• During telophase II1. a nuclear envelope forms around each set of

chromosomes2. the spindle breaks down, and the cell goes through

cytokinesis.

• The result of meiosis is four haploid cells.

THE STAGES OF MEIOSIS

THE SUMMARY OF MEIOSIS• One Diploid Germ Cell goes through 2 divisions

and genetic shuffling to create 4 genetically different haploid cells.

2 Divisions &Genetic Shuffling

COMPARING MITOSIS AND MEIOSIS• The processes of mitosis and meiosis are similar but meet

different needs and have different results. • It basically boils down to differences; one in purpose, one in

product.

Process Purpose Product

Mitosis new cells that are used during growth, development, repair, and asexual reproduction of somatic (body) cells.

two genetically identical diploid cells.

Meiosis makes cells that enable an organism to reproduce sexually, happens only in reproductive structures (sexual reproductive organs).

four genetically different haploid cells that contain half the genetic information of the parent cell.

FACT CHECK: COMPARING MITOSIS & MEIOSISRESTATE 2 MAIN FEATURES THAT MAKE MITOSIS AND MEIOSIS DIFFERENT,

THEN RESTATE THE MAIN DIFFERENCE BETWEEN THE PROCESSES.

• Mitosis1. Makes new cells that are

used during growth, development, repair, and asexual reproduction

2. Produces two genetically identical diploid cells

• Meiosis1. Makes cells that enable an

organism to reproduce sexually and happens only in reproductive structures (sexual reproductive organs).

2. Produces four genetically different haploid cells

A main difference between meiosis and mitosis is that in meiosis, genetic information is rearranged leading to genetic variation in offspring

GENETIC VARIATION

• You know that genetic variation is advantageous for a population.

• Genetic variation is made possible by sexual reproduction.

• The three key contributions to genetic variation are:1. crossing-over2. independent assortment3. random fertilization

Leave space below each of these themes!

GENETIC VARIATION

Crossing-Over• During prophase I, homologous chromosomes line

up next to each other.

• Each homologous chromosome is made of two sister chromatids attached at the centromere.

• Crossing-over happens when one arm of a chromatid crosses over the arm of the other chromatid.

GENETIC VARIATION

Crossing-Over• The chromosomes break at the point of the

crossover, and each chromatid re-forms its full length with the piece from the other chromosome.

• Thus, the sister chromatids of a homologous chromosome no longer have identical genetic information.

DEFINE SOME TERMS

• This is a set of homologous chromosomes.

• One is from mom and one is from dad.

• Being homologous means that the chromosomes are the same size and shape and most importantly have the same genes.

• They are also referred to as homologues.

• Both chromosomes make a tetrad.

• Remember that each chromosome that goes through meiosis exists as a pair of sister chromatids.

A A

Mom

b b

a a

Dad

B B

DEFINE SOME TERMS

• Tetrad = 2 homologous chromosomes.• One is from mom and one is

from dad.• They are also referred to as

homologues.

• Sister chromatids.• Chromosome/homologue• Centromere

A A

Mom

b b

a a

Dad

B B

DEFINE SOME TERMS

• How many… • Chromosomes• Centromere• Chromatid• Homologues• Tetrads

• …appear here?• Options:

• A: 1• B: 2• C: 3• D: 4

A A

Mom

b b

a a

Dad

B B

– B. 2– B. 2– D. 4– B. 2– A. 1

CROSSING OVER.• Not to be confused with a

basketball move…• During prophase

homologues cross over each other and exchange DNA.

• With this exchange of DNA is the exchange of genes.

A A

Mom

bb

a a

Dad

B B

CROSSING OVER.

• The exchanged DNA contains different versions of the same gene.• Say blond or black for the

hair color gene.

• In most cases the cross-over event creates new chromatids that different combinations of versions of the gene than before.

A A

Mom

b

a a

Dad

BB

CROSSING OVER.

• Now there exists 4 chromatids with 4 different gene combinations.

• This is extremely important in creating variable gametes.

• What are the new combinations of genes?• Ab• AB• ab• aB

A A

Mom

b

a a

Dad

BB b

1 2 3 4

CROSSING OVER.

• Compare the new chromatids with original chromatids.

• Notice that each are slightly different.

• This cross over ultimately creates 4 possible gene combinations and genetic diversity.

• This is not that big of a deal with just two genes but imagine if this was happening with every chromosome and there were 50 genes swapped each time.

• This is why brothers and sisters (siblings) don’t look alike.

A A

Mom

b

a a

Dad

BB b

A A

Mom

b b

a a

Dad

B B

A A

Mom

b

a a

Dad

BB b

Anaphase IAnaphase IIAnaphase II

CROSSING-OVER PRODUCTS

• Now instead of having 2 of the exact same chromatids there are 4 and the genes versions are shuffled.

• For example.• Gene A is black hair• Gene a is blond hair• Gene B is brown eyes• Gene b is blue eyes.• The possible gametes could be?

• Black hair, brown eyes (AB)• Black hair, blue eyes (Ab)• Blond hair, brown eyes (aB)• Bond hair, blue eyes. (ab)

A A

Mom

b

a a

Dad

BB b

GENETIC VARIATION2. Independent Assortment• During metaphase I, homologous

pairs of chromosomes line up at the equator of the cell.

• There is no rule stating which “color” lines up on which side.

• The two pairs of chromosomes can line up in either of two equally probable ways.

• This random distribution of genes on different homologous chromosomes during metaphase I & metaphase II of meiosis is called independent assortment.

EQUATOR

INDEPENDENT ASSORTMENT

• When chromosomes line up along the equator during metaphase I there is NO law that makes them do it any particular way.

• It equally probable for the tetrad to line up like this.

• Or this.• The chromosomes having the

ability to assort themselves independently from one another creates another layer of even more genetic diversity.

b BB b

Mom Dad

M mm M

Mom Dad

EQUATOREQUATOR

INDEPENDENT ASSORTMENT

• What are the allele combinations for this arrangement?

• You look at each individual chromatid!

• This is one possible outcome for a gamete.

A A

b

a a

BB b

Mom Dad

Y Y

M

y y

mm M

Mom Dad

INDEPENDENT ASSORTMENT

• This is another

A

b BB b

Mom Dad

Y

M mm M

Mom Dad

INDEPENDENT ASSORTMENT

• This is another

b BB b

Mom Dad

M mm M

Mom Dad

INDEPENDENT ASSORTMENT

• This is another

b BB b

Mom Dad

M mm M

Mom Dad

INDEPENDENT ASSORTMENT

• And for the other chromatid.• This can happen for each and

every chromatid.• The possibilities are very high

and varied.

b BB b

Mom Dad

M mm M

Mom Dad

INDEPENDENT ASSORTMENT

• And what if the bottom tetrad switched arrangement.

• Now the possibilities for the arrangement become even more numerous.

• & this is for just 2 chromosomes.

• Humans have 23 in each set.• That means that there exits

234 (279841) possible combinations for just one gene!

b BB b

Mom Dad

M mm M

Mom Dad

INDEPENDENT ASSORTMENT

• The key to this is that genes on different chromosomes can sort themselves out into different gametes independently of other genes.

• There is no rule to how they arrange themselves on the equator during metaphase I.

b BB b

Mom Dad

M mm M

Mom Dad

GENETIC VARIATION, CONTINUED

Random Fertilization• Fertilization is a random process that adds

genetic variation.

• The zygote that forms is made by the random joining of two gametes.

• Women use just one egg per cycle but male product millions of genetically diverse sperm.

• Because fertilization of an egg by a sperm is random, the number of possible outcomes is squared.

CONSIDER EVEN MORE DIVERSITY

• Why can there be so much diversity?• Think about all the random combinations

of genes in a completely planned mating.• Imagine just one gene, color blindness

let’s say.

b

b

B

B

B

b

b

B

This is for just one gene That only has two versions.Think of all the possibilities if there were more genes or if there were more than 2 versions of the gene.Because fertilization is random, there exist x2 possibilities for each gamete combination.

SUMMARY

• What is the point of meiosis?• Examples:

• Humans have 46 chromosomes in normal cells.• If we have too many, we have serious problems.

• Down syndrome is a result of having 47 chromosomes (3 - #21 chromosomes).

• Meiosis produces gametes with how many chromosomes?• What would happen if meiosis produced daughter cells

like mitosis?

SUMMARY

• During meiosis, a diploid cell goes through two divisions to form four haploid cells.

• Mitosis produces cells that are used during growth, development, repair, and asexual reproduction. Meiosis makes cells that enable an organism to reproduce sexually and it only happens in reproductive structures.

• Three key contributions to genetic variation are crossing-over, independent assortment, and random fertilization.

PRACTICE

Dad’s Germ Cell

X

Y

Z

X

Y

Z

x

y

z

x

y

z

X

y

Z

X

y

Z

X

y

z

X

y

z

Mom’s Germ Cell What do mom & dad look like?

Crossing Over Homework = 25pts.Key:Symbol = TraitX = Circlex = squareY = 4 straight hairy = 4 curly hairZ = 4 polka dots insidez = 4 square inside

Dad’s Germ Cell

X

Y

Z

X

Y

Z

x

y

z

x

y

z

X

y

Z

X

y

Z

X

y

z

X

y

z

Mom’s Germ Cell

What could their babies look like?Crossing Over Homework = 25pts.Key:Symbol = TraitX = Circlex = squareY = 4 straight hairy = 4 curly hairZ = 4 polka dots insidez = 4 square inside

Crossing Over

QUESTIONS?

• Meiosis Practice• CW: Complete the cut-out/matching activity on the back

of the crossing-over/random fertilization activity.• You are not concerned with “dominant” or “recessive” yet.

• Check this off with me before starting (completing) the meiosis notes WS.

• Complete the Meiosis Notes WS. Due tomorrow at the end of class.

WORKSHEET ANSWERS

9. Meiosis is the process that creates 4 haploid gametes diploid sex cell.10. In the beginning of human meiosis the germ cell is diploid, containing 2 sets of 23 chromosomes. Another way to write this is 2n = 46.11. In the end, 4 haploid gametes are formed. Another way to write these gametes is 1n = 23.12. Male gametes are called sperm and female gametes are celled eggs or ovum.13. In meiosis, genetic variation is caused by 2 events; Crossing-over in the prophase I stage of meiosis, and independent assortment in the metaphase I & metaphase II stages.

14. Independent assortment is: the random distribution of genes on different homologous chromosomes during metaphase I & metaphase II of meiosis 15. Compare and contrast mitosis and meiosis.

16. If one germ cell of a dog (2n = 78) undergoes meiosis and another cell undergoes mitosis, how many cells will result in each process and how many chromosomes will each resulting cell contain? • Dog cell mitosis = 2 cells, 78 chromosomes• Dog cell meiosis = 4 cells, 39 chromosomes

17. This is one sex cell that is about to enter metaphase I of meiosis I. This cell is (diploid or haploid). Label the following features, including the number of each in the cell:

a. Set of homologous chromosomes. 2b. Chromosomes: 4c. Chromatid: 8d. Tetrad: 2e. Which set is from mom and which is from dad?

18. These are the resulting gametes that are formed from one sex cell going through meiosis in telophase II. These cells are (diploid or haploid). Label the following features, including the number of each in each cell:

1.Homologous chromosomes. 02.Chromosomes: 23.Chromatid: 24.Tetrad: 0

19. Below, Draw One Tetrad Going Through The Cross-over Event. In Three Separate Stages (Beginning, Middle, And End) Show How The DNA Is Exchanged, Resulting In 4 Unique Chromosome Banding Patterns. Illustrate Each Chromosome In A Different Shade To Show Which One Is Mom’s And Which One Is Dad’s Original DNA.