Chap. 9 Cell Division

Chapter 9: Cell Division

The cellular basis of inheritance

https://www.youtube.com/watch?

Cells from cells

Cells: Why divide?

Repair, growth, replace old cells (uses

Mitosis)

Cells from cells

Two types of Reproduction

Asexual: single parent

• _{Single cell replicates genetic material and splits into} two or more identical new cells

• _{Offspring are genetically identical to parents and each} other

• _{Many unicellular organisms: bacteria, protists}

Sexual: 2 parents

• _{Sex cells from parents combine to form offspring}

• _{Offspring are genetically unique from each other and} parents

Dividing Onion Cells

Cell Wall/ Membrane Nuclei

Chromosomes and cell division

Eukaryotes have genes (DNA) in their nucleus:

chromatin: thin

uncondensed DNA and protein

chromatids: condensed/coiled DNA and protein

– _{As a cell prepares to divide}

DNA condensing from chromatin form to a

Chromosomes duplicate before

cell division

Duplicated chromosomes are called

sister

Chromosomes

Made of DNA and protein (histones)

Come in pairs (one each from mother

and father)

Different species have diff. numbers:

Humans: 46 chromosomes (23 pairs) Chimps: 48 chromosomes (24 pairs) Fruit Fly: 8 chromosomes (4 pairs)

Cell Cycle with MITOSIS

Cell cycle

: period of

growth and division in

a cell

 _{Replicate somatic}

(non-sex) cells (ex: skin cells)

 _{Some cells divide once}

a day; some almost never

 _{10-20 hours for}

Cell Cycle with MITOSIS

copies genes/DNA)

3 parts: G1, S, G2

Mitotic (M) Phase (when the cell is dividing)

Mitosis (nucleus and chromosomes divide):

P,M,A,T (Prophase, Metaphase, Anaphase, Telophase)

Interphase

*90% of a cell's life

3 Parts:

G1: Growth/ normal cell functions

S: DNA synthesis (DNA is copied)

Prophase

• _{longest, busiest phase}

• _{chromatin condenses to visible chromosomes}

• _{nuclear membrane & nucleolus disappear}

• _{Centrioles move to poles}

• _{spindle fiber (microtubule) forms at}

Metaphase

"meta"= middle

spindles move

Anaphase

Chromosomes split

Sister chromatids pulled to

opposite poles along spindle – Each chromatid is now an

Telophase

• _{Reverse prophase}

• _{Nucleolus, nuclear membrane}

reappear

• _{Spindle fibers disappear/ break}

apart

Cytokinesis

Divides cytoplasm

Same time as telophase

Animal cell: pinches in at

middle

– _{Like a drawstring bag}

Controlling Cell Division

G1, G2 checkpoints (to make sure cell cycle is progressing properly):

• _{Need for new cells}

• _{Undamaged DNA}

• _{Proper growth}

There is another checkpoint between Meta- and

Controlling Cell Division

Cancer

Cell division despite "stop" signals at cell cycle

checkpoints

Uncontrolled cell division

Example: defective p53 gene

in many cancer cells allows cell to ignore checkpoints.

(The p53 gene encodes a tumor suppressor

Cancer (continued)

Benign tumor

: abnormal mass of mostly normal

cells

Malignant tumor

: mass of cancer cells

– May spread to other sites (metastasis)

Treatment:

– _{Chemo/ radiation therapy (kills cancer cells)}

– _{Targeted treatment: try to only affect cancer cells}

Chromosomes

Karyotype: picture of all chromosomes in

a cell Humans have

?

Chromosomes

Homologous chromosome pair

(homologs)

1 from mother/ 1 from father

Same shape, size, centromere position

Same order of genes controlling same inherited characteristic i.e. eye color, hair color

*May have different version of a gene (different

*23

chromosome =

sex chromosomes

XY male (non homologous pair) XX female (homologous pair)

Diploid and Haploid cells

Diploid=2n

[ in humans: 2n=46]

Cell has two full sets of chromosomes (both homologs

of each pair)

Somatic cells (almost all human cells)

Haploid=n

[humans: n=23]

Single set of each chromosome

Diploid and Haploid cells

When haploid egg & sperm join through

fertilization

the fertilized egg, (called a

Different types of cell division:

Mitosis vs. Meiosis

Mitosis

: forms 2 identical diploid (2n) somatic

cells

WATCH THE ANIMATION HERE!

Meiosis

Form haploid (1n) gametes (sperm or egg)

Chromosome # in each cell cut in half: 2n  n

2 Nuclear divisions Stages:

– _{Interphase (growth, DNA replication)}

– _{Meiosis I: recombine and split homologous chromosome} pairs

Prophase I

• _{DNA coils and condenses} • _{Spindle fibers appear}

• _{Nuclear membrane & nucleolus disappear}

Synapsis: each chromosome (made of 2 sister chromatids) lines up next to its homolog to form a tetrad

Crossing over: Chromosomes twist around each other and exchange genetic material

– _{Causes genetic recombination}

– _{Chromosomes are mixes of mother’s and}

Metaphase I

Anaphase I

Homologous chromosomes (Tetrads) separate, move to opposite poles.

Chromosomes are still intact

Telophase I & Cytokinesis I

• _{Chromososmes arrive at poles} • _{cytoplasm divides (cytokinesis)}

• _{results in 2 haploid daughter cells- 23 chromosomes}

each

• _{each chromosome is still made of two sister chromatids} • _{Each daughter cell has a mix of mother’s and fathers}

Meiosis II

*no Interphase, no DNA

replication

Prophase II

– _{chromosomes coil again} – _{spindles form again}

Metaphase II

– _{Chromosomes attached to}

spindle fibers line up in middle

Anaphase II

– _{sister chromatids separate and}

Telophase II

– spindles dissolve

– _{nuclear membrane re-forms around chromatids (now}

called chromosomes)

Cytokinesis

Humans

2 sets of 23 chromosomes.

(homologous pairs)

23 chromosomes per cell

(2 chromatids each)

23 chromosomes per cell

Meiosis forms gametes differently

in males and females:

Male (Spermatogenesis

)-

– _{Even division}

Female (

Oo

genesis)

- Unequal division:

1 ootid (egg) and 3 polar bodies formed

– _{ootid= female gamete (can become}

fertilized)

– _{polar bodies eventually dissolve}

Fertilization: Sperm & Egg

Chromosomes, genes, and

inheritance

Why are offspring genetically

different than either parent cell?

*There are several causes of genetic variation in gamete formation and fertilization:

1.Crossing over (Prophase I)

2. Independent Assortment of Homologous chromosomes (Metaphase I/Anaphase I)

3. Random split of sister chromatids (Anaphase II)

4. Random which female egg becomes ootid (Telophase II)