Cell Reproduction:

Mitosis and Meiosis

Big Idea # 3

Genetics and Information Transfer

Cell

Tissue

Organism

Population

Ecosystem

How does the process effect each level of

organization?

Terms

• Anaphase• Binary fission• Cancer• Cell cycle• Cell plate• Checkpoint • Chromatin• Chromosome• Crossing over

• Cleavage furrow• Cyclin• Cyclin-dependent kinase

(Cdk)• Cytokinesis• Density dependent

inhibition• Diploid

• G0

• G1

• G2

• Gamete • Genome• Haploid • Homologous

chromosomes• Interphase• M phase• Meiosis

• Metaphase• Mitosis• MPF• PDGF• Prophase• S phase• Sister chromatids • Somatic cell• Telophase

BIG Idea # 3 Genetics and Information Transfer

3.A.2 – The Cell Cycle Is A complex Set of Stages That Is Highly Regulated With Checkpoints, Which Determines

the Ultimate Fate of the Cell

Cell Cycle

3.A.2 – The Cell Cycle Is a Complex Set of Stages

• Mitosis passes a genome from the parent cells to daughter cells– Mitosis occurs after DNA replication

– Produces two identical cells

– Growth, repair, asexual reproduction

– Continuous process with observable structures (order of the processes – replication, alignment, separation)

• Mitosis alternates with interphase

Genome

• All the genes of an organism• Genes – sections of DNA with code for making

proteins– Humans - 25,000 genes*,

46 chromosomes

– Average gene - 3000 nucleotides

Cell Division

• Prokaryote – circular DNA– Bacteria divide by binary

fission

– No coiling of DNA

– No proteins to manage the DNA

• Eukaryote = DNA coils into multiple chromosomes – DNA ‘managed’ by

proteins

• Plasma membrane grows inward

• Mitochondria and chloroplasts go through similar processes

Binary Fission in Bacteria

• DNA is making proteins most of the time

• Chromatin - DNA + proteins (histones)

• Before mitosis, chromatin is replicated; chromatin condenses, coils and folds into a chromosome

Eukaryotes - Chromatin

Chromosomes

• Chromosome = two sister chromatids connected by a centromere

• Chromatids are pulled apart into two new cells at the end of mitosis/meiosis

Homologous Pairs

Chromosomes• Each species has a characteristic number of

chromosomes– Human somatic cells have 46 (diploid)– Human gametes have 23 (haploid)

• Karyotype – arrangement of chromosomes

3.A.2 – The Cell Cycle Is a Complex Set of Stages

• After specialization, a cell enters a non-dividing state but may re-enter the cell cycle when given appropriate cues

• Interphase - three phases– Growth– Synthesis– Preparation for

mitosis

• Interphase - cell growth– Longest time of the cycle – Three subphases:

• G1 (“first gap”) growth

• S (“synthesis”) DNA is copied

• G2 (“second gap”) cell completes preparations for division

• G0 – some cells do not re-enter ‘S’

Cell Cycle

• The cell cycle is directed by internal control or checkpoints internal and external signals provide stop-and-go signs at the checkpoints.– MPF

– Platelet-derived growth factor (PDGF)

– Cancer results from disruptions of the cycle controls

– Cyclins and cyclin-dependent kinases control the cell cycle

3.A.2 – The Cell Cycle Is a Complex Set of Stages

• Frequency of cell division depends on the type of cell– Skin cells - frequent

– Liver cells do not divide unless damaged (Go)

– Nerve and muscle cells do not divide after maturity (permanent Go )

• Cell cycle is controlled by chemical signals: – Evidence: fuse a cell in S phase with a cell in G1, the G1

cell will start S– Evidence: fuse a cell in mitosis with one in interphase

causes the cell in interphase to start mitosis

Regulation of the Cell Cycle

Cell Cycle Control

• Checkpoints in cycle are control points– Checks to be sure all ‘steps’ are completed

– Hormone signals from outside also help control the cycle

• 3 checkpoints; G1, G2, and M phases

Cell Signaling

Cell Cycle Control

• G1 checkpoint (restriction point) is most important– Go signal = completes cell cycle and divides– No Go = cell exits cycle

Cell Cycle Control

• Rhythmic fluctuations of proteins controls the cycle– Kinases - activate or deactivate other proteins

• Constant amount

– Cyclins – levels fluctuate

• Kinases and cyclin form cyclin-dependent kinases (Cdks)

• Cyclin increases during interphase, then decreases during mitosis

• MPF – composed of cyclin-Cdk– “Maturation-Promoting Factor” triggers cell past G2

checkpoint to M phase

Cell Cycle Control

Internal and External Cues

• M phase checkpoint - ensures that the chromosomes are attached to the spindle at metaphase plate before anaphase begins so that daughter cells do not end up with missing or extra chromosomes– APF

• Mitosis is a continuous process:– Mitosis is ‘usually’ broken into four subphases:

• Prophase• Metaphase• Anaphase • Telophase

Prophase - Formation• Chromatin coils up to form chromosomes

• G2 checkpoint

– MPF builds to a peak

Metaphase - Alignment• Spindle fibers push the chromatids until they are all arranged

at the metaphase plate

• M – phase checkpoint

– APF builds up, MPF degrades

Anaphase - Separation• Centromeres divide

– Separates chromatids• Each chromatid is pulled toward the pole by spindle

fibers

Telophase:

• Cell elongates• Two nuclei envelopes begin to reform• Chromatin uncoils• Cytokinesis begins

• Animal cells:– Cleavage furrow - contractile ring of

actin and myosin forms

• Plants have cell walls – Cell plate - vesicles from Golgi

coalesce at the metaphase plate

– Plate enlarges until fused with the plasma membrane

Cytokinesis

• Density-dependent inhibition – Normal cultured cells divide until they form a

single layer

– Cells will grow to fill a gap

• Anchorage dependence – cells must be anchored (extracellular matrix)

Internal and External Cues

• Do not respond to density-dependent inhibition or anchorage dependence

• Do not stop dividing when growth factors run out• May be ‘immortal’

– Normal cells - 20 to 50 times in vitro

– HeLa cells - Henrietta Lacks

Cancer Cells

Terms

• Allele• Chromosome • Crossing over• Fertilization • Gamete • Gametogenesis• Gene• Locus

• Meiosis I• Meiosis II• Sex chromosome• Sexual reproduction• Synapsis• Tetrad• Zygote

• Meiosis, a reduction division followed by fertilization, ensures genetic diversity in sexually reproducing organisms– Ensures each gamete receives one complete haploid

(1n) set of chromosomes

– Homologous chromosomes are paired with one homologue originating from the maternal parent and one from the paternal parent. Orientation of the chromosome pairs is random with respect to the cell poles

– Separation of the homologous chromosomes ensures that each gamete receives a haploid set of chromosomes composed of both maternal and paternal chromosomes

– Homologous chromosomes may exchange genetic material via ‘crossing over’ which increases genetic variation

– Fertilization involves the fusion of two gametes increasing genetic variation in populations by providing for new combinations of genetic information in the zygote and restores the diploid number of chromosomes

Asexual Reproduction• Single parent• No genetic variation

– Cloning, plant cuttings

– Binary fission in bacteria

– Spores – plants, fungi

• Very rapid, energy efficient method• Few mutations - ‘bad’ or ‘weak’ genes also passed

along• Genes susceptible to environmental change

Sexual Reproduction

• Sexual reproduction:– Requires more energy – Slower– Gametogenesis - meiosis

• Genetic variation***• Survival of the fittest

• Somatic cell - any cell other than gamete• Gamete - sex cell, haploid

Homologous Chromosomes

• Homologues - same size, same genes• Same gene loci – location of gene on the chromosome• Autosome - non-sex chromosomes (22 pairs)• Sex chromosome – carries gene that determines

gender; dissimilar

Heredity: Meiosis

• Mitosis:

• 4 stages• 2 identical daughter cells• Diploid (2n)

• Meiosis:

• 8 stages• 4 non-identical cells• Haploid (1n)

Meiosis – Crossing Over• Prophase I:• Synapsis - homologous

chromosomes come together as pairs

• Tetrads• Crossing over may occur

Nondisjunction

• Chromatids fail to separate during meiosis• Gametes are ANEUPLOID (have incorrect number

of chromosomes)