Cells IVCells IV

MeiosisMeiosis

What is Meiosis?What is Meiosis?

A special type of cell division used only for A special type of cell division used only for the production of gametesthe production of gametes Gametes are the reproductive cells of an Gametes are the reproductive cells of an

organismorganism Typical mitosis cell division produces Typical mitosis cell division produces

genetically identical daughter cellsgenetically identical daughter cells Why wouldn’t this work for gametes?Why wouldn’t this work for gametes?

Gamete ProductionGamete Production Human somatic cells (those not used for Human somatic cells (those not used for

reproduction) have 46 chromosomesreproduction) have 46 chromosomes A chromatid has a complete set of an organisms DNAA chromatid has a complete set of an organisms DNA A chromosome has 2 copies of that setA chromosome has 2 copies of that set

This is called a “diploid number” of chromosomes (2x)This is called a “diploid number” of chromosomes (2x)

If gametes were made like somatic cells, through If gametes were made like somatic cells, through mitosis, the sperm and the egg would each have mitosis, the sperm and the egg would each have ______ chromosomes______ chromosomes

This means that upon conception, a zygote This means that upon conception, a zygote would have ______ combined chromosomes would have ______ combined chromosomes from its parentsfrom its parents

46

46

Fertilized Egg

?

Gamete ProductionGamete Production

As a result of the problem of excess As a result of the problem of excess chromosomes, gametes are produced in a chromosomes, gametes are produced in a different waydifferent way

MeiosisMeiosis Often called “reduction division”Often called “reduction division”

________________________ must be reduced, ________________________ must be reduced, producing haploid cellsproducing haploid cells

Haploid = ½ x the number of necessary chromosomesHaploid = ½ x the number of necessary chromosomes _____________ must be divided_____________ must be divided

ChromosomesChromosomes Segments of DNA found in our chromosomes contain genes

Each chromatid has a complete set of our genetic code, the chromosome, then, has 2 copies of that complete code

Since each of 2 parents donated chromosomes to us, they are said to come in pairs (23 pairs in humans). These pairs are said to be homologous pairs because they each have genes for the same traits. For example, a homologous pair might have the gene for eye color from the mother on one chromosome and a different gene for eye color from the father on the other.

The PhasesThe Phases Meiosis begins with the same type of Meiosis begins with the same type of

starting cell as mitosis (diploid cells)starting cell as mitosis (diploid cells) Meiosis undergoes the same phases as Meiosis undergoes the same phases as

meiosis (twice)meiosis (twice) Meiosis is split into 2 major parts:Meiosis is split into 2 major parts:

Meiosis I: reduction of chromosomes (by ½) Meiosis I: reduction of chromosomes (by ½) by separating homologous pairsby separating homologous pairs

Meiosis II: division of chromatids (just as in Meiosis II: division of chromatids (just as in mitosis)mitosis)

The Phases: Meiosis IThe Phases: Meiosis IProphase I: formation of chromasomes, crossing-over (explained later)

Metaphase I: homologous chromosomes line up next to their homologues, spindle fibers begin to attach to centromeres

Anaphase I: separation of homologous chromosomes (reduces the # by ½)

Telephase I: parent cell begins to divide into 2 daughter cells (cleavage furrow in animal cells, cell plate forms in plant cells)

2 daughter cells are made in Meiosis I

1 Chromosome from each homologous pair can be found in each daughter cell

The Phases: Meiosis IIThe Phases: Meiosis II

Prophase II: chromosomes remain intact after first division on both new cells

Metaphase II: chromosomes now line up single file as they do in mitosis; spindle fibers attach to centromeres

Anaphase II: chromatids are divided with one copy of the DNA going each way

Telophase II: each daughter cell from the first division now begins to divide

The final product of meiosis is 4 daughter cells (all gametes, all haploid)

This would make 4 sperm or 1 egg/3 polar bodies (discarded)

Crossing OverCrossing Over In prophase I of meiosis, homologous In prophase I of meiosis, homologous

chromosomes allow genes to detach and chromosomes allow genes to detach and reattach (even on the homologue)reattach (even on the homologue)

This means that the chromosomes a This means that the chromosomes a person receives from his/her parents are person receives from his/her parents are not passed in their exact forms on to not passed in their exact forms on to his/her children, but rather some mixture his/her children, but rather some mixture of the twoof the two

Crossing over contributes to genetic Crossing over contributes to genetic diversitydiversity

Crossing OverCrossing Over

Mitosis vs. MeiosisMitosis vs. Meiosis Generates somatic Generates somatic

daughter cellsdaughter cells Generates 2 cellsGenerates 2 cells Generates genetically Generates genetically

identical copiesidentical copies Undergoes 1 divisionUndergoes 1 division No crossing overNo crossing over Homologous pairs Homologous pairs

line up single fileline up single file

Generates gamete Generates gamete daughter cellsdaughter cells

Generates 4 cellsGenerates 4 cells Generates genetically Generates genetically

unique copiesunique copies Undergoes 2 divisionsUndergoes 2 divisions Crossing over in Crossing over in

prophase Iprophase I Homologous pairs Homologous pairs

line up together in line up together in metaphase Imetaphase I