Cell Reproduction

• Understand why cells reproduce

• Describe the structure of the chromosome

• Describe the cell cycle

• Describe each phase of mitosis

• Distinguish between mitosis and meiosis

• Describe the process and phases of meiosis

Terms • Anaphase

• Asexual reproduction

• Autosomes

• Cell cycle

• Cell plate

• Centromere

• Centrioles

• Centrosome

• Cleavage furrow

• Chromosome• Crossing over• Cytokinesis• Diploid• Gamete • G1

• G2

• Haploid• Histone • Interphase • Meiosis

• Metaphase• Mitosis• Microtubules• Prophase • Restriction point• S phase• Sexual reproduction • Sex chromosome• Spindle • Somatic cell• Synapsis • Telophase • Tetrad

Why Cells Must Reproduce

• Cells are limited in size due to:

– Surface area to volume ratio: S.A./volume• Volume increases 8x faster than surface area• The area in the ‘middle’ of the cell cannot get

nutrients in or wastes out because there is not enough surface area for diffusion

– Amount of DNA – limited amount of instructions, cell cannot keep up with demand

Why Cells Must Reproduce• To grow an organism must produce more cells• Each new cell must have an

accurate copy of the DNA (genome)

• Mitosis – Growth – Replace cells that wear out– Replace damaged cells– Asexual reproduction

Purposes of Mitosis

• Asexual reproduction:Asexual reproduction:

– Unicellular organisms

• Amoeba, paramecium

– Multicellular organisms • Corals, sponges, plants

• Genome – cell’s genetic information

– Prokaryote = single, long DNA molecule (?)• E. coli – 1700 genes

• Binary fission – bacteria cell division

– Eukaryote = lots of DNA• Humans – 3.2 b nucleotides

– 25,000 genes (est. HGP ’09)– Coils up to form 46 chromosomes

Genome

Chromosomes• Genome – genes

• Genes - sections of DNA that are the code for making the cell’s proteins

• Chromatin – DNA + histones (proteins) ‘at work’

– Making proteins

ChromatinChromatin

Chromosomes

• Chromatin coils up to become chromosomes

– Chromosomes are ‘gene packages’

Chromosomes Chromosomes

• Each chromosome = two sister chromatidschromatids (identical DNA)

• CentromereCentromere = connects the two chromatidschromatids

Chromosomes

• Each species has a characteristic number of chromosomeschromosomes

– Human somatic cells (body cells) have 46

(diploid number)

– Dogs = 78, goldfish = 96

– Human gametesgametes (sperm or eggs) have 23 (haploid)

Karyotype

• KaryotypeKaryotype – arrangement of chromosomes

– Picture taken during metaphase

– Size

– Centromere location

– Staining pattern

Normal male Normal male

Cell Cycle

• Life history of the cell

• Interphase

• Mitosis

• Cell growth• 90% of cell cycle

– Three subphases:

• G1 (“first gap”) growth

• S (“synthesis”) DNA is copied

• GG22 (“second gap”) cell completes preparations for division

Cell Cycle - Interphase

Cell Cycle - Interphase

• Some cells may enter G0

– Don’t return to mitosis

– Liver, nerve, muscle

• Mitosis – division of the nucleus and chromosomes

• Cytokinesis – division of the cytoplasm and organelles

Cell Cycle – M-phase Cell Cycle – M-phase

• Mitosis is a continuum:

– For description, mitosis is usually broken into five subphases:

• Prophase

• Prometaphase

• Metaphase

• Anaphase

• Telophase

Cell Cycle – M-phaseCell Cycle – M-phase

• Late interphase (G2)

• DNA has been copied

– ‘S’ phase

– Chromatin – not chromosomes yet

• Centrosomes have been duplicated

– Microtubules (cytoskeleton)

– Will help form the spindle

Cell Cycle - InterphaseCell Cycle - Interphase

• Prophase - the chromosomes are coiled, with sister chromatids

• The nucleoli disappear

• The mitotic spindle begins to form

• Centrosomes move toward opposite poles

• Spindle fibers (microtubules) begin to grow between the centrosomes

Cell Cycle – M-phaseCell Cycle – M-phase

• Late Prophase - nuclear envelope disintegrates so that spindle fibers can attach to the chromosomes

• Kinetochores – special regions of the spindle, from each pole attach to the centromere

Cell Cycle – M-phaseCell Cycle – M-phase

• Metaphase - spindle fibers push the sister chromatids until they are all arranged at the metaphase plate

• Shortest phase of cycle

Cell Cycle – M-phaseCell Cycle – M-phase

• Anaphase - centromeres divide, separating sister chromatids

• Each chromatid is pulled toward the pole to which it is attached by spindle fibers

Cell Cycle – M-phaseCell Cycle – M-phase

• Telophase - cell continues to elongate

– Two nuclear envelopes begin to reform

– Chromosome uncoils

– Nucleolus reforms

• Cytokinesis begins

Cell Cycle – M-phaseCell Cycle – M-phase

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 12.9

• Animal cells:

• Cleavage furrow – microfilaments of actin and myosin form and contract like a drawstring

• Contraction of the ring pinches the cell in two

Cytokinesis Divides Cytoplasm

• Plants have cell walls

• Cell plate - vesicles from the Golgi coalesce at the metaphase plate

– Plate enlarges until fused with the plasma membrane

Cytokinesis Divides Cytoplasm

Cell Cycle Control

• Checkpoints in the cycle are controls

– Chemical signals

– Checks to be sure all steps are completed

– Cells also receive signals from outside (ex. Growth hormones)

• 3 checkpoints; G1, G2, and M phases

• G1 - Restriction point

• Frequency of cell division varies with cell type– Skin cells divide frequently; exercise– Liver cells do not divide unless damaged– Nerve and muscle cells do not appear to divide

after maturity

• Much is not yet understood– How and why are cells preprogrammed to become

__?– How and why do some cells become cancerous?

Cell Cycle Control

Cell Cycle Control

• G1 checkpoint (restriction point) is most important.– Go signal = completes cell cycle and

divides.– No go = cell exits cycle; switches to

non-dividing state, GG00 phase phase

• DensityDensity-dependent inhibitiondependent inhibition

– Cultured cells normally divide until they form a single layer

– Cells will grow to fill a gap– At high densities, not

enough growth factor and nutrient for all cells

Cell Cycle Control

• AnchorageAnchorage dependence - dependence - cells must be cells must be anchored to a substrate (extracellular matrix)anchored to a substrate (extracellular matrix)

• Cancer cellsCancer cells do not respond to density-dependent inhibition or anchorage dependence

Cell Cycle Control

• Cancer cells do not stop dividing when growth factors are depleted

– Manufacture their own growth factors (?)

– Abnormality in the signaling pathway

– Problem in the cell cycle control system

• If cancer cells stop dividing, they do so at random, not the normal checkpoints

Cancer Cells Not Under Cell Cycle Controls

• Cancer cells may divide indefinitely if they have a continual supply of nutrients

• Normal cells divide 20 to 50 times (in vitro) before they stop, age, and die.

– Cancer cells may be “immortal”• Cells (HeLa) from a tumor removed from a

woman (Henrietta Lacks) in 1951 are still reproducing in culture.

Cancer

Cancer• A cell in a tissue undergoes a transformation

converts it from normal to cancer– Normally, immune system recognizes and

destroys transformed cells– Some cells escape destruction and

reproduce to form a tumortumor, a mass of abnormal cells

Cancer Tumors

• Two types: benign, malignant• BenignBenign – tumor remains at original site –

surgery• MalignantMalignant – cell leave original site, spread

to other organs• MetastasisMetastasis – tumor cells pop ‘loose’ enter

lymph

Cancer - Treatment

• Surgery

• Radiation and chemotherapy

• Interrupt cell cycle

– Mitotic spindle

– Block chemical controls

Meiosis

Cell Reproduction

That Produces Gametes

• Gametes (eggs or sperm) are produced only in gonads (ovaries or testes)

• Cells undergo meiosis meiosis in the in the gonadsgonads

– Produces four daughter cells; half the chromosomes of each parent (haploidhaploid)

• Fertilization - gametes fuse together; restores the number to 46. (diploiddiploid)

MeiosisMeiosis

Meiosis• Chromosomes are homologous pairs

– 23 pairs of homologous chromosomes• 22 pairs of autosomes

1 pair of sex chromosomes

– Same genes

– Requires two parts for every gene – one homologue from one parent and the other from the other parent

Meiosis

• During ‘S’ each homologue is copied

Meiosis

• Meiosis takes place in two steps

– Meiosis I

– Meiosis II

• Phases are the same but happen twice

– Prophase

– Metaphase

– Anaphase

– Telophase Interphase

Meiosis

• Prophase I

– Same as prophase of mitosis – except:

• Synapsis - homologous chromosomes pair up side-by-side

• Crossing over - tips of chromosomes may swap places

• Value of crossing over?Prophase I

Crossing over

Meiosis

• Metaphase I

– Chromosome pairs line up at the middle

Metaphase I

Meiosis

• Anaphase I

– Chromosome pairs separate and migrate to opposite poles

– In mitosis, chromatids separate and migrate to poles

Anaphase I

Meiosis

• Telophase I

– Chromosome pairs arrive at the pole

– Chromosomes uncoil

– Nuclear envelope forms

– Cytokinesis

Telophase I

Meiosis

• Meiosis II

– Prophase II

– Metaphase II

– Anaphase II

– Telophase II

– 4 new haploid cells called gametes are produced

Meiosis II

Meiosis

• Produces:

– 4 non-identical, haploid gametes

– Haploid gametes unite to form a zygote• Zygote undergoes mitosis to become a baby

– Random chance determines which chromosome you inherit from each parent

– Crossing over increases genetic variation

Comparison of Mitosis and Meiosis

• Mitosis – 2 identical cells– New cells are diploid– Growth, repair,

asexual reproduction– No synapsis or

crossing over– One cell division

• Meiosis – 4, non-identical cells– New cells are haploid– Sexual reproduction– Synapsis and crossing

over – Two cell divisions

Mitosis and Meiosis

• How are mitosis and meiosis alike?

• DNA is replicated

• Phases are the same

• Spindle forms

• Cytokinesis

• Cells are reproduced