Biology - Cell Division, Cancer, Meiosis

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

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