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Gamete Production Human somatic cells (those not used for reproduction) have 46 chromosomes A chromatid has a complete set of an organisms DNA A chromosome has 2 copies of that set This is called a “diploid number” of chromosomes (2x) If gametes were made like somatic cells, through mitosis, the sperm and the egg would each have ______ chromosomes This means that upon conception, a zygote would have ______ combined chromosomes from its parents
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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
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