Chapter 8 Part 2

Meiosis

Cancer is uncontrolled cell division

Cell cycle control is lacking in cancer cells• The cell cycle control system ensures that the

stages occur in order and only when the previous stage has been successfully completed.

• G1 checkpoint – cell enters G0 or, if DNA is damaged and cannot be repaired, apoptosis occurs.

• G2 checkpoint – mitosis will occur if DNA has replicated properly. Apoptosis will occur if DNA is damaged and cannot be repaired.

• M checkpoint – mitosis will not continue if chromosomes are not properly aligned

• Apoptosis – programmed cell death. Cell pulls away from other cells, starts to break apart, and the pieces are engulfed by white blood cells or neighboring cells. This allows organisms to take shape – tadpoles losing their tail, fingers and toes in humans losing webbing as an embryo.

Development of cancer (carcinogenesis)• Mutations due to environmental assaults can

result in abnormal growth• Tissues with a high rate of division are more

susceptible• If a cell with a mutation is allowed to divide, it

becomes more abnormal with each generation• Cancers are classified according to tissue of

origin– Carcinomas – tissues that line organs– Sarcomas – muscle, bone, cartilage– Leukemias - blood

Cancer cell characteristics• Lack differentiation – nonspecialized, do

not contribute to the functioning of a body part

• Have abnormal nuclei – enlarged and may have abnormal number of chromosomes

• Form tumors• Undergo angiogenesis and metastasis

– Angiogenesis – formation of new blood vessels, these supply nutrients and oxygen to the cells

– Metastasis – cancer cells begin new tumors far from the primary tumor

Meiosis reduces the chromosome number

Homologous chromosomes separate during meiosis• During meiosis I, homologous chromosomes line up side

by side (synapse)• After cell division, the daughter cells are haploid• Interkinesis – time between meiosis I and meiosis II• During meiosis II, sister chromatids are separated

resulting in 4 haploid daughter cells• Daughter cells mature into gametes (sperm and egg)• Fertilization – bringing together of sperm and egg,

resulting in diploid zygote

Synapsis and crossing over occur during meiosis I

• Synapsis – homologous chromosomes line up side by side

• Crossing over – homologous chromosomes (homologues) exchange genetic material. Increases genetic variability of the daughter cells

Sexual reproduction increases genetic variation

• Homologues line up independently at equator

• At the end of meiosis any short chromosome can end up with any long chromosome

• Fertilization – gametes carry different genetic information due to independent assortment and crossing over. The same couple can have offspring that are very different genetically.

• Advantages of genetic variation– Some offspring have a better chance of

survival that others– In warmer environments, dogs with short hair

may have an advantage over dogs with long hair

Meiosis requires meiosis I and meiosis II

• Prophase I – chromosomes have duplicated, homologues pair during synapsis and crossing over occurs

• Metaphase I – homologues align independently at the equator

• Anaphase I – homologues separate and are pulled toward the poles

• Telophase I – nuclei reform

• Interkinesis – between meiosis I and meiosis II – chromosomes still have sister chromatids

• Prophase II – cells have one chromosome from each pair of homologues

• Metaphase II – chromosomes align at the equator

• Anaphase II – sister chromatids separate and become daughter chromosomes

• Telophase II – nuclei reform, cytokinesis takes place

• Daughter cells – 4 haploid daughter cells

Life cycles are varies

• Asexual reproduction– Prokaryotes– Offspring are genetically identical to parent

• Sexual reproduction– Eukaryotes– Two parents, involves meiosis and mitosis,

offspring are genetically different from parent

3 possible life cycles• Haploid life cycle

– Algae– Adult is haploid– Asexual reproduction if environment is stable– Sexual reproduction occurs when environment is unstable,

produces zygote that can survive bad weather

• Alternation of generations life cycle– Plants– Meiosis produces haploid gametes– Fertilization results in diploid cells

• Diploid life cycle– Animals– Haploid egg and sperm produced– Fertilizes to produce diploid zygote

Meiosis can be compared to mitosis

Meiosis• 2 nuclear divisions• 4 daughter cells• Haploid• Genetically differentMitosis• 1 nuclear division• 2 daughter cells• Diploid• Genetically identical

Chromosome anomalies can be inherited

Nondisjunction causes chromosome number anomalies

• Polyploid – 3 or more complete sets of chromosomes. This is common in many crops and fruits.

• Aneuploid – different number of a particular chromosome.– Monosomy – one of a particular chromosome (2n – 1)– Trisomy – three of a particular chromosome (2n + 1)

• Nondisjunction – During meiosis I –

homologues do not separate and both go to the same daughter cell

– During meiosis II – sister chromatids do not separate and both go to the same daughter cell

– Occurs in plants and animals. Lethal most of the time in animals, but those with trisomy are more likely to survive that monosomy. Sex chromosome aneuploids have a better chance of surviving that do autosomal aneuploids.

Chromosome number anomalies can be observed

Trisomy 21 (Down syndrome)

• Autosomal chromosome

• Easily recognizable physical and mental characteristics

• Chances of a woman having a child with Down syndrome increases with age.

• Most children with Down syndrome are born to mothers younger than 40, because that age group is having the most babies.

• Turner syndrome (XO) female with one X

• Before and after surgery to remove neck webbing.

• Trisomy (XXX) female with 3 X chromosomes. Lack symptoms.

• Klinefelter syndrome (XXY) male. Lack some male characteristics, very long arms and legs.

• (XYY) males appear normal

Chromosome structure anomalies can also be observed

• Can lead to various syndromes

• Deletion – loss of part of a chromosome

• Duplication – a particular chromosome segment appears more than once in the same chromosome

• Inversion – reversed sequence in part of a chromosome

• Translocation – movement of a chromosome segment from one chromosome to another

Syndrome examples• Williams syndrome – deletion

• Cat’s cry syndrome – deletion

• Down syndrome – 5% of the time due to translocation

• Certain types of cancer - translocation

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