Feb 2 Lecture Notes

Tues Feb 2

Necrosis – when cells die not by choice; cells die by being damaged (poison, lack of oxygen); does NOT use ATP; cell swells up and then blows up; dead cells are ingested by white blood cells; EX Spider bite
Apoptosis – cells are programmed to die (how cells die when they want to die) (cell suicide); cells are given signal to kill themselves off; cell breaks up into little bits and then dead cells are ingested by neighboring cells; uses ATP; EX cells of embryo die in order to allow full grown organism to have individual fingers

What’s difference between cancel cell and normal cell?
Normal cells will divide until they’re touching other cells
Contact inhibition – cells will NOT divide if they’re touching other cells; cells will NOT enter S-phase of cell cycle
Normal, healthy cells have contact inhibition
Cancer cells do not have contact inhibition
Cancer cells divide constantly without growth factors
Cancer cells don’t possess proteins which stop replication
Cancer cells do not undergo apoptosis on schedule
Constant cell division/preparation for cell division accounts for morphology of cancer cells
Nuclei of cancerous cells are larger than normal cells; constantly using energy; nuclei has to replicate itself constantly, so nuclei is larger; cytoskeleton (actin and tubulin filaments make it up) has to break down so cytokinesis can happen; cancer cell is “blobbyer”

Why cells don’t divide all the time
Every time a cell with linear chromosomes (eukaryotes) divides, DNA molecules / chromosomes get shorter. Once chromosome gets short enough, apoptosis happens.

Definitions of Terms
Gametes – haploid reproductive cells (eggs and sperm)
Haploid – possessing one set of chromosomes per cell
Somatic cells – body cells not specialized for reproduction
Each somatic cell contains homologous pairs (two pairs of chromosomes) of chromosomes with corresponding genes. Each parent contributes one homolog.

What is meiosis?
Meiosis – consists of two nuclear divisions that reduce chromosome number to haploid. Unique to reproductive cells
DNA is replicated only once. (1st step)
Products are different from parent cell and from each other
Homologous pairs are going to cross over/swap parts (2nd step)
After parts are swapped, there will be two rounds of cell division
Will end up with gametes that only have half the number chromosomes of parent cell

Why is meiosis necessary for sexually reproducing organisms?
Have to reduce number of chromosomes by half otherwise there would be too much DNA in each cell
Why is sexual reproduction (as opposed to just mitosis) a good idea?
It waters down all mutations; increases diversity (don’t know what future holds. Being diverse helps an organism survive)

Mitosis vs Meiosis

Unlike mitosis:
In meiosis 1, homologous pairs of chromosomes come together and pair along their entire lengths
After metaphase 1, homologous pairs segregate; sister chromatids remain together until after metaphase 2

Meiosis 1

Prophase 1
DNA replicated – 2 sets of DNA; chromosomes condense into chromatin; condense into individual chromosomes
Swapping of bits between replicated pairs of chromosomes – crossing over
End up with chromosomes that are different than original chromosomes – contain part from other chromosome
Chromosomes are recombinant
Recombinant – Containing DNA from more than 1 organism

Metaphase 1
Pairs of chromosomes line up in center.
Anaphase 1
Pairs are pulled apart by microtubles and move to poles.
Telophase 1
Cytokinesis occurs (myosin)
Have 2 cells with complete sets of DNA (23 pairs)

Meiosis 2 – DNA NOT replicated

Prophase 2
NO COPYING OF DNA. Chromosomes condense.
Metaphase 2
Sister chromatids line up in middle
Anaphase 2
Sister chromatids pulled apart into individual chromosomes to opposite poles.
Telophase 2
Chromosomes gather into nuclei
Products: cells with HALF the number of chromosomes as regular cell (haploid cells)

Differences between mitosis and meiosis 2
Dna doesn’t replicate before meiosis 2
Meiosis 2 sister chromatids may not be identical because of crossing over in prometaphase 1
Number of chromosomes at equatorial plate in meiosis 2 is half number of those in mitosis

Problems with meiosis
Nondisjunction – homologous pairs fail to separate at anaphase 1; or sister chromatids fail to separate; or homologous chromosomes may not remain together
Results in down’s syndrome
If both chromosomes 21 homologs go to same pole, and resulting eggs is fertilized, it be trisomic for chromosome 21
Results in aneuploidy – chromosomes lacking or present in excess

If you had sample of tissue, how many cells would you see in metaphase?
Should see no cells in metaphase unless cells are cancerous and they’d be dividing constantly

Only time when you see cells in phase is when you look at tissue repairing injury, embyo

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