Meiosis and Sexual Reproduction Meiosis and Sexual Reproduction Production of Haploid Cells Variation in Genetic Material.

Meiosis and Meiosis and Sexual Sexual

ReproductionReproductionProduction of Haploid CellsProduction of Haploid Cells

Variation in Genetic Variation in Genetic MaterialMaterial

Sperm and EggSperm and Egg

Sperm trying to enter egg and leave packet of Sperm trying to enter egg and leave packet of genetic information – i.e. chromosomes containing genetic information – i.e. chromosomes containing DNA. DNA.

Only one sperm will be successfulOnly one sperm will be successful

For zygote to have same number chromosomes as For zygote to have same number chromosomes as parents, chromosome number must be halved parents, chromosome number must be halved when egg and sperm made when egg and sperm made

Halving chromosome number done by MEIOSIS Halving chromosome number done by MEIOSIS (produces haploid cells)(produces haploid cells)

Diploid number restored when sperm fertilizes eggDiploid number restored when sperm fertilizes egg

ReviewReview

What are the What are the following?following?

Evolution Evolution

Natural selectionNatural selection

Homologous Homologous chromosomeschromosomes

ChromatidsChromatids

AnswersAnswers Change in genetic makeup Change in genetic makeup

of species over time.of species over time.

Individuals with favorable Individuals with favorable traits are more likely to traits are more likely to survive and reproducesurvive and reproduce

Chromosomes similar in Chromosomes similar in size, shape and genetic size, shape and genetic makeupmakeup

Copied chromosomes still Copied chromosomes still attached by centromereattached by centromere

ReviewReview

What are the What are the following?following?

Haploid CellsHaploid Cells

Diploid CellsDiploid Cells

Spindle FibersSpindle Fibers

The steps of mitosisThe steps of mitosis

AnswersAnswers Cells with one set of Cells with one set of

chromosomeschromosomes

Cells with two sets of Cells with two sets of chromosomeschromosomes

A structure made of A structure made of microtubules - helps microtubules - helps organize and separate organize and separate chromosomeschromosomes

Prophase, Metaphase, Prophase, Metaphase, Anaphase, TelophaseAnaphase, Telophase

What would happen if the What would happen if the number of chromosomes number of chromosomes

weren’t reducedweren’t reducedOrganis

mDiploid Chrom. Number

# after 1

generation

# after 2 generati

ons

# after 3 generati

ons

# after 4 generati

ons

Fruit Fly 8 16 32 64 128

Garden Pea

14 28 56 112 224

Horse 64 128 256 512 1024

Formation of Haploid Formation of Haploid CellsCells

MEIOSIS

Meiosis IMeiosis I1st Meiotic Division

Before Meiosis 1, DNA Before Meiosis 1, DNA replicatedreplicated

11stst step of Meiosis 1 step of Meiosis 1

Chromosomes condenseChromosomes condense Watch it coil!

Nuclear membrane breaks Nuclear membrane breaks downdown

Homologous chromosomes pair Homologous chromosomes pair upup

Crossing over (parts of Crossing over (parts of chromatid on one homologous chromatid on one homologous chromosome exchanged with chromosome exchanged with corresponding part on other corresponding part on other homologous chromosome)homologous chromosome)

Animation of Prophase 1

Prophase 1Prophase 1

Meiosis 1, continuedMeiosis 1, continued

22ndnd phase of Meiosis 1 phase of Meiosis 1

Pairs of homologous Pairs of homologous chromosomes are moved chromosomes are moved by spindle fibers to cell by spindle fibers to cell equatorequator

Homologous Homologous chromosomes remain chromosomes remain togethertogether

This is when independent This is when independent assortment occursassortment occurs

Animation of Metaphase 1

Metaphase 1Metaphase 1

Meiosis 1, continuedMeiosis 1, continued

33rdrd phase of Meiosis 1 phase of Meiosis 1

Homologous Homologous chromosomes separatechromosomes separate

Chromosomes of each Chromosomes of each pair are pulled to pair are pulled to opposite poles by opposite poles by spindle fibersspindle fibers

CHROMATIDS DO NOT CHROMATIDS DO NOT SEPARATE AT THEIR SEPARATE AT THEIR CENTROMERES!CENTROMERES!

Each chromosome made Each chromosome made of two RECOMBINED of two RECOMBINED chromatidschromatids

Animation of Anaphase 1

Anaphase 1Anaphase 1

Meiosis 1, continuedMeiosis 1, continued

44thth stage of Meiosis 1 stage of Meiosis 1

Individual Individual chromosomes gather chromosomes gather at each of the polesat each of the poles

Cytoplasm dividesCytoplasm divides Both cells contain 1 Both cells contain 1

chromosome from each chromosome from each pair of homologous pair of homologous chromosomeschromosomes

Animation of Telophase 1 and cytokinesis

Telophase 1 and Telophase 1 and cytokinesiscytokinesis

Meiosis IIMeiosis II2nd Meiotic Division

Meiosis IIMeiosis II

11stst Phase of Meiosis II Phase of Meiosis II

New spindle fibers New spindle fibers form around the form around the chromosomeschromosomes

Animation of Prophase II

Prophase IIProphase II

Meiosis II, continuedMeiosis II, continued

22ndnd phase of Meiosis II phase of Meiosis II

Chromosomes line up Chromosomes line up along the equator and along the equator and are attached at their are attached at their centromeres to spindle centromeres to spindle fibersfibers

Animation of Metaphase II

Metaphase IIMetaphase II

Meiosis II, continuedMeiosis II, continued

33rdrd phase of Meiosis II phase of Meiosis II

Centromeres divideCentromeres divide

Chromatids (now called Chromatids (now called chromosomes) move to chromosomes) move to opposite poles of cellopposite poles of cell

Animation of Anaphase Animation of Anaphase IIII

Anaphase IIAnaphase II

Meiosis II, continuedMeiosis II, continued

44thth phase of Meiosis II phase of Meiosis II

Nuclear membrane Nuclear membrane forms around each set forms around each set of chromosomes. of chromosomes.

Spindle fibers break Spindle fibers break downdown

Cells undergo Cells undergo cytokinesiscytokinesis

There are now 4 haploid There are now 4 haploid cellscells

Animation of Telophase Animation of Telophase II and cytokinesisII and cytokinesis

Telophase II and Telophase II and cytokinesiscytokinesis

IMPORTANT!IMPORTANT!

The purpose The purpose of of meiosis meiosis is is to produce to produce cells with half cells with half the genetic the genetic material as material as the parent the parent cellcell

IMPORTANT!IMPORTANT!

The purpose The purpose of of mitosis mitosis is to is to produce cells produce cells with same with same amount of amount of genetic genetic material as material as the parent cellthe parent cell

Meiosis and Meiosis and Genetic Genetic

VariationVariation

Independent Independent Assortment & Assortment &

Crossing Over Crossing Over

Genetic VariationGenetic Variation

Meiosis = rapid Meiosis = rapid generation of new generation of new

genetic combinationsgenetic combinationsHow genetic variation How genetic variation

arises arises Independent Independent

AssortmentAssortment

Crossing OverCrossing Over

Random Fertilization – Random Fertilization – Random combination Random combination of 2 gametes of 2 gametes

Why don’t they all look Why don’t they all look alike?alike?

Why is Genetic Variation Why is Genetic Variation Important?Important?

Evolution!Evolution! Pace of evolution Pace of evolution

increases as genetic increases as genetic variation increasesvariation increases

Genetic recombination Genetic recombination increases evolutionincreases evolution

33rdrd type is produced – type is produced – not identical to either not identical to either parentparent

Genetic variation can Genetic variation can bring out good traits or bring out good traits or not so successful traitsnot so successful traits

Gamete FormationGamete Formation

Sperm production = Sperm production = spermatogenesis – spermatogenesis – meiosis results in meiosis results in production of 4 spermproduction of 4 sperm

Egg production = Egg production = oogenesis – meiosis oogenesis – meiosis results in production of 1 results in production of 1 egg cell and 3 polar egg cell and 3 polar bodies because bodies because cytoplasm divides cytoplasm divides unequallyunequally Polar bodies diePolar bodies die Egg livesEgg lives

Egg = Egg = OvumOvum

Modeling Crossing Modeling Crossing OverOver

Each person in group Each person in group needsneeds

4 paper strips – 2 of one 4 paper strips – 2 of one color, 2 of another color – color, 2 of another color – taped together (loosely) taped together (loosely) to represent chromatids to represent chromatids joined by centromeresjoined by centromeres

Pen or pencilPen or pencil Scissors Scissors TapeTape A textbook (you should A textbook (you should

end up with 4 textbooks end up with 4 textbooks in a group)in a group)

ProcedureProcedure On two strips of the same On two strips of the same

color, write “A” , “B”, and color, write “A” , “B”, and “C” –one of the two “C” –one of the two homologous chromosomeshomologous chromosomes

On two strips of another On two strips of another color, write “a” ,“b” , “c” color, write “a” ,“b” , “c” this is another the two this is another the two homologous chromosomeshomologous chromosomes

Use scissors and tape to Use scissors and tape to demonstrate crossing over demonstrate crossing over between chromatids of between chromatids of homologous chromosomeshomologous chromosomes

Modeling Crossing Modeling Crossing OverOver

What do the letters “A”, “B” and “C” and “a” , What do the letters “A”, “B” and “C” and “a” , “b” , and “c” represent?“b” , and “c” represent?

Why are the chromosomes you made homologous?Why are the chromosomes you made homologous?

Compare the number of types of different Compare the number of types of different chromatids (combinations of “A”, “B”, “C”, “a”, chromatids (combinations of “A”, “B”, “C”, “a”, “b”, and “c”) before crossing over and after “b”, and “c”) before crossing over and after crossing over.crossing over.

How does crossing over relate to genetic How does crossing over relate to genetic recombination?recombination?

During what phase of meiosis does crossing over During what phase of meiosis does crossing over occur?occur?

Modeling Independent Modeling Independent AssortmentAssortment

In your groups of 4, line up homologous pairs as In your groups of 4, line up homologous pairs as the chromosomes would line up during Metaphase the chromosomes would line up during Metaphase I. Do this on top of two textbooksI. Do this on top of two textbooks

How many different arrangements can you make?How many different arrangements can you make?

What will happen during Anaphase 1? Will the 2 What will happen during Anaphase 1? Will the 2 cells which are the result of Meiosis one contain cells which are the result of Meiosis one contain the same genetic information?the same genetic information?

Pull books apart to represent cytokinesisPull books apart to represent cytokinesis

How does independent assortment relate to How does independent assortment relate to genetic recombinationgenetic recombination

Modeling Meiosis II Modeling Meiosis II

You should now have 2 cells – each with 4 double You should now have 2 cells – each with 4 double chromosomes in them (there will be more or less chromosomes in them (there will be more or less chromosomes, depending on how many people are in chromosomes, depending on how many people are in your group)your group)

Line up double chromosomes as in Metaphase II. Line up double chromosomes as in Metaphase II. Use another textbook to add to your “cytoplasm”.Use another textbook to add to your “cytoplasm”.

Snip tape between the centromeres so that the Snip tape between the centromeres so that the single chromosomes can travel to opposite poles. single chromosomes can travel to opposite poles. Pull books apart to model cytokinesisPull books apart to model cytokinesis

Do the same thing with your second cellDo the same thing with your second cell

How many cells do you have as a result of Meiosis? How many cells do you have as a result of Meiosis? Compare the chromosomes of your gametes. What Compare the chromosomes of your gametes. What do you notice?do you notice?