CELL DIVISION MITOSIS & MEIOSIS

Cell Cycle Interphase Mitotic 2 distinct phases S Chromosome duplication G2 What's the most important event of interphase?

What is significant about DNA in the S and G2 phases? All chromosomes are duplicated That means: two copies of each chromosome What are the copies called? Sister chromatids Do they contain identical genes? Yes, but… What about meiosis?

Some terminology When do each of these occur? Chromatin Homologous chromosomes Chromatid Double-chromatid chromosomes Single-chromatid chromosomes When do each of these occur?

THE STEPS OF MITOSIS Interphase Prophase Metaphase Anaphase Telophase (actually, this is not part of mitosis itself) Prophase Metaphase Anaphase Telophase

Interphase Nuclear envelop intact Mitosis Interphase Onion root tip Whitefish blastula Nuclear envelop intact Chromosomes duplicated but not apparent DNA loose, uncondensed, called chromatin 2 nucleoli visible in onion root tip Nucleoli not visible in whitefish blastula

Prophase Double-chromatid chromosomes evident Mitosis Onion root tip Whitefish blastula Double-chromatid chromosomes evident Chromatin becomes super-coiled & compact Nuclear envelop breaks down Centrioles migrate toward poles of cell forming the spindle Nucleoli disappear in onion root tip

Double-chromatid chromosomes

Mitosis Metaphase Onion root tip Whitefish blastula Double-chromatid chromosomes line up on equatorial plate of cell The spindle fibers attach to the chromosomes at the centromeres

Anaphase Spindle fibers pull chromatids apart Mitosis Anaphase Onion root tip Whitefish blastula Spindle fibers pull chromatids apart Centromeres are pulled apart Groups of single-chromatid chromosomes move opposite poles of the cell

Telophase Groups of single-chromatid chromosomes reach poles of cell Mitosis Onion root tip Whitefish blastula Groups of single-chromatid chromosomes reach poles of cell Nuclear envelop begins to reform 2 new daughter cells formed Cytokinesis begins with appearance of cell plate Cytokinesis begins with appearance of cleavage furrow Nucleoli reform

Returning to Interphase Mitosis Returning to Interphase Onion root tip Whitefish blastula Cytokinesis completes Chromosomes ‘disappear’ as interphase chromatin reforms 2 new daughter cells enter G1 phase of the cell cycle

A SUMMARY OF MITOSIS

Mitosis Questions What does “diploid” mean? We have __ total chromosomes. In the term 2n, what does “n” stand for in us? In a gypsy moth? Why does mitosis occur? Major functions? In what cells (general term) does mitosis occur?

Mitosis Questions 6. What are chromatids? 7. What happens in Anaphase to result in each new cell receiving duplicate parental DNA? 8. In a species whose diploid number is 224, what would its sperm/eggs contain?

Why is this duplication of parental DNA necessary? Something to do with passing on genetic information?

Why undergo mitosis at all? Something to do with cells getting damaged, old, lost? Something to do with the organism growing, infant to adult?

SOMATIC CELLS What are typical body cells called? DNA Mitosis These cells divide continuously The new cells receive an exact copy of all the parent cell’s: DNA What is the process of somatic cell duplication called? Mitosis

What are these somatic cells? Diploid or 2n What does this mean? They contain the full number of chromosomes in pairs How many in humans? 46 23 pairs

Mitosis occurs only in somatic cells What about sex cells? Called gametes Eggs and sperm Produced in ovaries or testes Gametes are not diploid (2n) Instead, they are haploid (n)

Our haploid (n) number is 23 So our eggs and sperm have how many chromosomes? Half the number Why?

Fertilization is the union of an egg and a sperm If the egg and sperm were both diploid, what would the fertilized egg (zygote) be?

It would be a genetic mess! Instead, gametes are haploid (n). Egg and sperm both have exactly half the number of chromosomes of somatic cells At fertilization, n + n = 23; 23 + 23 = 46!

Note how mitosis and meiosis differ: Number of divisions? Number of chromosomes? Number of products?

Meiosis ! Somehow somatic cells (2n) in our ovaries or testes must produce gametes (n) Meiosis is the cell division process that enables the transformation from 2n to n

How is meiosis more complicated than mitosis? Gametes must contain precisely half the diploid number of chromosomes They must contain one of each homologous pair of chromosomes Remember karyotypes? Each chromosome has a partner They come in pairs One from mom One from dad

Human Karyotype

Which pair of chromosomes in us in not homologous? It’s the 23rd pair in males, the XY pair Remember what homologous means? All our other pairs of chromosomes are homologous

How many divisions does meiosis have? What are the divisions called?

1 2 3 4 One pair of homologues to start DNA is replicated Meiosis 1 = 2n ---> n Meiosis 2 = chromatids separate into 4 products

In males, how many progeny are produced? Typically 4 viable sperm are produced following each Meiosis 2

In females, how many progeny are produced? Just one viable ovum (egg) is produced, plus 3 small polar bodies

Crossing over When does it occur? Only during Prophase of Meiosis 1 Homologous chromosomes get together in temporary tetrads Overlap (cross over) and trade their DNA Why is this a good thing to do, generally?

Meiosis Summary Meiosis 1 DNA replication takes place A parent cell produces two daughter cells each with one member of each original pair of homologous chromosomes (to create haploid daughter cells) Crossing over may occur

Meiosis Summary Meiosis 2 There is no more DNA replication The chromatids of each chromosome separate and each daughter cell divides At the end of Meiosis 2, there are 4 daughter cells from each parent cell. Each daughter cell has half the number of chromosomes as the parent cell

Meiosis Questions The cells produced in meiosis are _____ (haploid or diploid?) Sex cells are called ________. _____ are produced by males, ____ by females. 3. What’s crossing over and why is it important?

Meiosis Questions 4. In what meiotic stage does crossing over occur? 5. Why are sex cells n, not 2n? 6. If a species’ diploid number is 50, what is n?