Monday, 29 August 2011

3.24 Mitosis

Understand that division of a diploid cell by mitosis (cell division) produces two cells which contain identical sets of chromosomes.
Cell division => growth/increase in number of cells

a. 
The Diploid number is the number of chromosomes in the nucleus.
This is sometimes abbreviated to the symbol 2n, for humans the number is 46. So, 2n=46 for humans, for cats 2n=38. In the process known as mitosis a cell is divided to two cells - each with a nucleus. If we look at the nucleus we will find that each one will have a diploid nucleus. We often say these cells are identical or we can call it as daughter cells. (they have the same number of chromosomes and they have the same set of chromosomes.)

b. How are copies made and How do they separate?

Mitosis is done through a process called DNA replication, in this process each chromosome under goes a copying process to form another identical copy of itself of the same genes and alleles. The two copies are held together by a structure around the center region known as the centromere. We refer to them as a pair of chromatids. The process of DNA replication takes place inside the nucleus while its still inact. This is known as the interphase of the cell cycle.
C. The stages of mitosis
Observing a cell through a microscope, we will see a nucleus has a spherical structur and would be unable to see the chromosomes. It is during the interphase that DNA replication occurs.
Prophase is where the breaking down of a membrane occurs and the chromosomes become visible, and they are visible as a pair of chromotids. 
The nucleus will be gone after prophase, leaving the cell with protein molecules called spindle. These extend from one pole of a cell to another. The pair of chromatids will soon join on one of the spindle fibres at the centromere.
Metaphase is when the pair of chromatids are attached to a spindle fibre by the centromere, characteristic of the metaphase is that the chromatid is in the middle across the equator of the cell.
In the Anaphase the spindle fibre shortens pulling chromotids into different direction (apart) and they move to the poles of the cell and seperates.
Telophase is the last phase of mitosis, in which the nucleus reforms around the chromosomes at either ends. We see the formation of two nuclei at opposite ends of the cell.
Cytokinesis  is the process in which the cell splits into two which is not part of mitosis. Here we see the cytoplasm of the cell split and eventually forms two cells.


Wednesday, 24 August 2011

3.16 DNA and Genetic Information

describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases: adenine (A) with thymine (T) and cystosine (C) with guanine (G).

If we look at a section of the double helix we will see bases that are close together and there are four different types of bases: adenine (A) with thymine (T) and cystosine (C) with guanine (G). In the molecule the bases are what holds the double helix together. And they are held together by pairing between adenine -thymine and guanine -cystosine. These are known as the base pairs.
If we look on one side of helix you will see the sequence of A,C,T,G,A,A,C,C,A,G. This is the 'order of the vase' => and its this order that we call the GENE.
Gene can be defined as the order of the bases and the number of the bases. This is for the construction of the protein in the cytoplasm which gives us the characteristic.

3.15 Genes

Understand that Genes is a section of the molecule of a DNA.

A Section of the molecule of DNA is called a gene which carries the characteristic of an organism which could be bloog group or petal colour. The information is passes to the cytoplasm, and in the cytoplasm the genetic information is transformed into a protein and it is the protein that controls the production of the characteristic of an organism.

Gene (Nucleus) =============> Protein (Cytoplasm)

3.14 Chromosomes

Recall that the nucleus of a cell contains chromosomes on which genes are located.

Chromosomes contains genetic information in an cell, which is in a nucleus of a cell.
Chromosomes are made up of a molecule called DNA (Deoxyribonucleic acid) and this forms a shape known as double helix. Sections of these molecules is called genes.
One Chromosome will have possible thousands of genes. Each gene carries the information for the construction of a protein. This gives the characteristic associated to the gene. For instance, blood group.
Different animals have different number of chromosomes: cat has 38, a chicken has 78, chimpanzees have 42 and we have 46 per cell.
Chromosomes operates in homologous pairs , and the homologous nature depends on the length of the homologous pairs.

If we look at a gene, the position is known as a gene loci. If  we go to the same positiion on the homologous pair we find the same gene loci. = same gene. Therefore we have 2 versions of each gene for one characteristic. This version is called alleles.


Saturday, 13 August 2011

3.1 Sexual and Asexual reproduction

Describe the differences between sexual and asexual reproduction.

  • Organisms that shows sexual reproduction show sexes (male/female) whereas organisms that show asexual reproduction has no sexes.
  • Organisms that shows sexual reproduction HAS cells called GAMETES in them - in male organisms these take the form of sperm cells and in the female organisms the egg cells. In plants the sperm cell is often known as the pollen grain and the egg cell is known as the ovule. Organisms that show asexual reproduction has NO GAMETES.
  • The type of cell division that produces gametes is called MEIOSIS. Meiosis has a number of effects one of them is half the chromosomes in the gamete cell. In humans there are 46 chromosomes per cell. However, in gamete cells there are only 23 (this is halved by the process of meiosis). In asexual reproducing organisms there are no meiosis but rather MEITOSIS in eukaryotic cells binary fission in prokaryotic bacterial cells. In this process the number of chromosomes is maintained constantly and are identical.
  • In sexually reproducing population there is a process of fertilization (where the male gamete meets the female gamete) on the other hand, since they have no gamete cells there will be no fertilization happening in asexual reproducing population.
  • If we examine the population of sexual reproductive organisms we will find a broad variation and in asexual productive organisms there will be small variations due to mutation. By enlarge they are identical, this is known as a clone.
Question: [sexual reproduction] Can different organisms with opposite gametes mate? Why or why not? e.g. can you take a gamete from a male donkey and put it with a gamete from a female tiger and produce healthy offspring?