Biology  - Cell Division, Cancer,  Meiosis

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Exploring Cell Divison 

• Responsible for growth
• Humans start with one cell and then cells divide at a rapid pace during pregnancy and continue after birth
• Is most rapid during childhood and slows down during adulthood

Cell division

• After 5 divisions, the number of cells would be 32. Because 2 × 2 × 2 × 2 × 2 = 32, this can also be expressed as 25. After 10 divisions, the number of cells would be 1024, or 210
• Basic terms 
  • DNA - particularly for packaging and distribution 
  • Chromosome - once the cell is duplicated
  • Chromatids - 2 of these make up the “X”
  • Centromere - used to hold together the chromatids
• Chromosomes 
  • Different organisms have different amounts
  • Humans have 46
  • Chimpanzees have 48
  • Ducks have 80
  • Ferns have 1262
  • The number of chromosomes doesn’t determine the complexity 
  • During cell division, they will split apart so each of the daughter cells gets one copy
  • When a cell has a full complement of chromosomes it is called diploid
  • Sperm and egg cell contain only half the number of chromosomes and are called haploid 

Examining the Cell Cycle and Mitosis 

• The sequence of events from one division to the next is called the cell cycle

• Cell cycle in 3 phases 

  • Interphase
  • Where prepares to undergo cell division
  • Longest stage in the cell cycle
  • Cell grows and accumulates nutrients need for division 
  • Mitosis 
  • Dividing genetic material 
  • Every somatic cell undergoes this process
  • Except for reproductive cells which undergo meiosis

• 3 purposes for mitosis 

• GROWTH: Somatic cells undergo the process of mitosis in order to make more cells, which allows for the overall growth of the organism. Cells cannot simply grow in size; the distance of the nucleus from all parts of the cytoplasm must be kept small for efficient communication.

• REPAIR: Mitosis occurs in order to repair damaged tissue by replacing cells. If an organism receives a cut or scrape or a sunburn, cellular division will occur to heal the skin.

• ASEXUAL REPRODUCTION: In single-celled organisms, mitosis is a way to reproduce.

• Prophase 

  • Sister chromatids join to form chromosomes and the intersection of those is called a centromere
  • chromosomes condense and become visible
  • nuclear membrane begins to dissolve
  • centrioles used to move the chromosomes, migrate towards opposite ends of the cell (“the poles”)
  • spindle fibres (cellular “tow ropes”) begin to form from the centrioles

• Metaphase 

  • Chromosomes line up in the  equator (middle) of cell 
  • Single fibres connect to the centromeres of chromosomes

• Anaphase 

  • Simple fibres begin to retract 
  • This exerts a force on the sister chromatids that pull them apart 
  • Two halves move to opposite sides of the cell

• Telophase 

  • chromatids reach the opposite pole
  • genetic material begins to re-condense
  • a nuclear membrane begins to form around each set of chromatids
  • cell membrane begins to pinch and form two cells
  • the process of cytokinesis begins

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• Cytokinesis 

  • Final phase and it divides the cytoplasm into two producing two fully independent separate cells 
  • During cytokinesis in animal cells, the membrane pinches inward, forming a wall between the two halves
  • Eventually, this wall closes up, dividing the cell into two equal parts
  • Plants undergo a slightly different process, building up a new cell wall between the two halves of the nearly divided cell

Understanding Cancer

• Sometimes DNA can mutate leading to uncontrolled cell growth (cancer)
• Most cells divide through communication with similar cells, but cancer cells divide on their own, regardless of what other cells are doing around them
• Cancer cells divide at a faster rate than normal cells. 
• Your body has specialized cells called immune cells that detect and destroy bacteria and some kinds of cancer cells
• Unfortunately, sometimes your immune cells are unable to detect and destroy cancer cells because they are “camouflaged” as normal cells
• Tumours are mass of cancer cells that grow and divide 
• Some are benign (non-harmful)
• If cancer cells dislodge from the original tumour site, cancer can spread throughout the body through the circulatory or lymphatic systems and create new cancer colonies (a process called metastasis)
• In this case, the tumour is a harmful or malignant tumour. If caught early enough this cancer may be able to be treated 

Investigating Meiosis and Possible Errors 

• The process of mitosis produces identical copies of the parent cell, but sometimes an identical copy is not desirable
• Organisms that reproduce sexually, like humans, combine the genetic material of two parents to produce an offspring
• If each parent were to give a diploid cell (containing 2n chromosomes) to the offspring, then the offspring would start out with 4n (2n + 2n) chromosomes
• When that generation grew up and reproduced, their offspring would have 8n
• In only a few generations, the number of chromosomes in the cells would be in the trillions, and the cell would have to be as big as a house just to hold it all
• Clearly, this is not desirable, so reproductive cells undergo a process of division that does not duplicate the genetic material, but instead halves it to create sex cells or gametic cells, this process is called meiosis
• In males the process is referred to as spermatogenesis (the formation of sperm cells); in females, the process is referred to as oogenesis (the formation of egg cells)

Meiosis 

• Gametic cells must be haploid; that’s to say, they must have half the number of chromosomes as somatic cells
• In humans, this means that one sperm cell contains 23 chromosomes and one egg cell contains 23 chromosomes
• In order to create a haploid cell, the process of meiosis involves two divisions rather than just one
• As in mitosis, there is an interphase phase prior to meiosis in which the genetic information duplicates

Errors in cell division

• If errors occur during the division of a somatic cell, such as a skin cell or a liver cell, it may not cause much harm because there are many other cells of the same type to keep the organism functioning properly
• However, if something goes wrong in a reproductive cell during meiosis, the resulting fertilized egg could contain an improper amount of genetic material
• Since every subsequent somatic cell is a descendent of that original fertilized egg, each will contain the same mistake
• This could inhibit the organism from functioning properly
• The failure of chromosomes to divide correctly during meiosis is called nondisjunction
• Cells that lack genetic information or have too much information may not function properly

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