August 29, 2011

3.24 Mitosis

3.24 Understand that division of a diploid cell by Mitosis produces two cells which contain identical sets of chromosomes

What is Mitosis?
Mitosis is a form of cell division that results in the increase in the number of cells.

  1. Begin with a single cell
    • The number of chromosomes in this original cell is referred to as its Diploid number (2n)
    • All animals have a different Diploid number (Humans, 2n = 46)
  2. The cell divides into two daughter cells
    • These cells are identical to the mother cell
    • 1. Each of these cells has a Diploid nucleus (the same number of chromosomes as the mother cell)
    • 2. They have the same set of chromosomes as each other

How are the copies of chromosomes made?

DNA replication
  1. The process takes place inside the nucleus while it is still intact; before any separation has gone underway. This resting stage is known as the interphase stage of the cell cycle
  2. Each chromosome undergoes a copying process to form an identical copy of itself
    • It has all the same Genes and Alleles
  3. These two copies are held together by a structure around the centre by what’s called a centromere.
  4. These two chromosomes held together by a centromere are referred to as a pair of chromatids

How do they separate?

Cell Division
  1. The first sign a cell is entering mitosis is when we see the breakdown of the nuclear membrane (this is known as the Prophase)
    1. The chromosomes then become visible as a pair of Chromatids
  2. A network of Protein molecules (the Spindle and Spindle fibres) extend from one pole of the cell to the other (this is the Late Prophase)
  3. The pair of chromatids work their way towards the middle of the spindle and join onto a spindle fibre by the centromere (the Metaphase)
  4. The Anaphase is where the spindle fibre shortens to pull each chromatid in an opposite direction and ultimately they ultimately separate and each one will rest in the opposing poles.
  5. The final phase is known as the Telophase when the nuclei begin to reform around the chromatids at each pole of the cell. There are now two nuclei
  6. And finally a phase called the Cytokinesis is where the cell splits in two, but this particular stage is not regarded as part of the Mitosis Cycle.
The image below visually explains the process

August 24, 2011

3.16 DNA and Genetic Information

3:16 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 cytosine (C) with guanine (G)

Chromosomes are likely to contain thousands of genes
Each gene contains a double helix (that is a pair of Helices that appear to be parallel to one another)

The Helices are known as the sugar-phosphate backbone
In the centre of these Helices there are four types of bases, known as adenine, thymine, cytosine and guanine. These bases are holding together the two helices. They are hold together by pairing up the four bases; A with T & C with G, these are known as the base pairs and are always found in DNA.
The order of these base pairs defines the gene. For example take the order of the bases shown on the right of below image: AGTGAACCAG. This order is called the Gene, so by changing the order, you theoretically change the gene.
We can therefore define a gene as being a combination of the order of the bases (ATGC) and the number of bases on one side of the helix. And this code somehow constructs specific proteins in the cytoplasm that somehow gives us the characteristics. 

3.15 Genes

3.15 Understand that a gene is a section of a molecule of DNA

A section of DNA is called a gene
Genes carry information which forms the characteristic of organisms e.g. blood group, petal colour, etc. (different gene for different characteristic)

The Genes are located in the chromosomes which are held in the Nucleus.
The characteristic information is passed to the Cytoplasm.
In the Cytoplasm the genetic information is transformed into a Protein, this protein then controls the production of its characteristic.

Information flow:
Gene (nucleus) -> Protein (cytoplasm)

3.14 Chromosomes

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

1. What are chromosomes, and what are they made of?
Chromosomes are Genetic information contained within the nucleus of a cell

Here you can see a pair of human chromosomes
Chromosomes are composed of a molecule known as DNA (Deoxyribonucleic acid), this DNA forms a shape known as the double helix [indicated in yellow in the above picture]

Sections of this DNA molecule are called Genes [see 3.15], one chromosome will have 1000’s of genes. Each gene carries the information for the construction of a Protein [see 3.16] the protein controls the characteristics associated with the gene (e.g. blood group)

DNA (Gene) -> Protein -> Characteristic

2. How many chromosomes?

Different organisms all have a different number of chromosomes
For example: a Cat has 38, a Chicken 78, a Chimp 42 and Humans have 46

3. Genes operate in pairs known as homologous pairs

This diagram illustrates some of the homologous pairs within an organism
Their Homologous nature and pairing is based upon the equal length of the chromosomes

If a gene is selected from one of the chromosomes in a homologous pair, its gene loci (position at which it is located in the chromosome) is the same in the pairing chromosome.

In a pair of chromosomes there are 2 versions of the same gene for one characteristic, one found in each chromosome but at the same gene loci. These versions are called Alleles.

August 14, 2011

3.1 Reproduction

3.1 understand the difference between sexual and asexual reproduction
An animal can be either Sexual or Asexual each type has it's own distinct characteristics as listed below

Sexual reproducing creatures

  • They have a sex/gender, so can be identified as either male or female.
  • They produce cells called 'Gametes', the sperm cell in males, and the egg cell in females
  • To produce these Gametes, a type of cell division called Meiosis is used, which is A type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell.
  • Fertilisation occurs, where opposite Gametes fuse together
  • There is a broad variation in the individuals (i.e. humans are very different from each other)
Asexual reproducing creatures:

  • There are no Sexes or Genders
  • There are no Gametes
  • Asexual reproduction involves Mitosis or Binary Fission (Note: Mitosis is used in Sexual reproducing creatures, but not for reproduction)
  • No fertilisation because there are no Gametes
  • There is little variation between individuals and this is due to mutations all others are "identical" also known as "clones"