Cell Division

Cells divide for a few reasons:

• growth
• repair
• asexual reproduction
• gamete production (for sexual reproduction)

Organisms have two types of cell division to cater for these different reasons: Mitosis and Meiosis.

MITOSIS

This type of cell division creates two daughter cells that are identical clones of the parent cell (unless a mutation occurs). We do this for growth and repair. Some organisms use this to create clones (asexual reproduction).

MEIOSIS

Organisms that use sexual reproduction rely upon meiosis to make gametes (sex cells). Sex cells need to be haploid (half the number of chromosomes), so they make a cell with the right number of chromosomes during fertilisation. Variation during meiosis can be inherited, as these cells are the start of a new life.

COMPARE AND CONTRAST

DNA Replication and Gene Expression

Sections of DNA code for different genes. DNA is an ideal molecule to carry this inherited information, because it can be replicated.

We need to know what the base sequence of a new strand of DNA, if we know the base sequence of the original strand of DNA.

We also need to understand that this is where a lot of mistakes can happen. Just like copying a computer file, errors can occur when DNA is replicated. A change to the base sequence is called a mutation.

Some mutations are benign. This means there is no effect on the gene. The same amino acids are made, even with the error in the base sequence. This is like making a minor spelling mistake in a sentence.

Other mutations cause changes to the base sequence. These mutations can result in new alleles for the gene. They can also lead to complications such as cancer, where the change causes the cell to grow and reproduce uncontrollably.

To understand the effect of mutations, it is useful to know about how the DNA is "read" by the cell. This is called gene expression and results in protein synthesis.

For a complete overview of DNA structure and replication, check out this video:

And this one gives a complete overview of gene expression:

Cells, Chromosomes and the Genetic Code

As living things, we are made up of cells. Inside those cells is a nucleus. Inside the nucleus of each cells are structures called chromosomes. Chromosomes contain our genetic code. The genetic code is a sequence of bases on a molecule called deoxyribonucleic acid (DNA). Sections of this DNA code for specific traits. These sections are called genes. There are often different versions of the same gene, called alleles. You got one allele from each parent. Alleles differ in their exact base sequence (genetic code), which gives variation (differences).

He patai? Do you have questions? Check out the appropriate video for a more detailed explanation:

What is DNA?

What are Chromosomes?