CELL DIVISION AND GENETICS

Chromosomes normally occur in pairs. Cells may have paired or unpaired chromosomes. When a cell has paired chromosomes it is diploid. If the chromosomes are unpaired, the cells is haploid. In humans, each cell should have 23 pairs (46) chromosomes. Thus the cells are diploid. Only gametes are haploid.
22 prs are referred to as autosomes and the last

ChromosomesPrd.gif
1 pr are called gonosomes or sex chromosomes.
Chromosomes are formed when the chromatin network becomes organized and compact.

chromosomes_crossing_over_large.jpg
A pair of chromosomes = homologous - they code for the same set genetic traits.
Individual pairs of codes = alleles i.e. Genes on sister-chromatics of a homologous pair of chromosomes that code eg. for eye colour.


THE CELL CYCLE


The cell has a cycle comprising of a growing phase followed by a dividing phase = interkinesis and cytokinesis.
During Interkinesis the cellular activities continue as per diagram.
CellCycle.jpg


Cell division is required for growth, repair of wounds, replacement of old and worn-out tissue and reproduction.

In lower organisms reproduction can occur by simple binary fission, accomplished through the process of mitosis. This means that the organisms resulting from such reproduction are replicas of the original organism. This is termed asexual or vegetative reproduction.

However, in many of such organisms, under unfavourable conditions, pairs of them fuse and the nucleus would undergo meiosis when conditions are favourable again. This would lead to some degree of genetic variation.

In higher organisms sexual reproduction occurs which renders each offspring genetically different from the other unless they have developed from the same zygote [identical twins, triplets etc.] For sexual reproduction to occur, gametes have to fuse. In order to maintain the chromosome complement, and not double it every time when gametes fuse, the paired chromosomes are separated during gamete formation. This is accomplished during meiosis.




Make a list of the differences between meiosis and mitosis.... process as well as results.


MITOSIS

The process of mitosis can be depicted as follows:


mitosis.jpg
PHASES

Prophase

Metaphase

Anaphase

Telophase






The result, 2 cells identical to the mother cell.

Importance of mitosis ----



MEIOSIS

Meiosis can only occur in diploid cells. It is termed 'reduction division' because the number of chromosomes are halved during this process. Two successive divisions occur, the second division, Meiosis II, is the same as any mitotic division.
The first meiotic division, as depicted below, is a reduction division

meiosis_1.jpg



THE SECOND MEIOTIC DIVISION:

meiosis_2.jpg




synapsis.jpg


Synapsis

during Meiosis I






Result of Crossing over -

daughter cells genetically different from mother cell.
A lot can go wrong during the process of synapsis when chromosomes