DNA

Year Nine
A brief summary of what DNA is, with details about its function and discovery.

 

Deoxyribonucleic acid is a substance which forms the genetic structure of all living organism. DNA is found in the chromosomes contained within the nuclei of living cells, where it is arranged in a double helix structure. The structure is made up of small sections called genes. Each gene helps to control the production of proteins within a cell, and through this they determine the physical characteristics of the organism. The individual strands of DNA are made up of combinations of six different chemicals: Adenine, Thymine, Cytosine, Deoxyribose sugar, Guanine and Phosphate. The chemicals are arranged in nucleotides, with a phosphate molecule first, then a sugar molecule, and then the “base” (adenine, thymine, cytosine, guanine). A DNA strand is formed when two nucleotides lock together. Cytosine is always opposite a guanine nucleotide, because the two molecules are shaped to perfectly lock together, making them “base pairs”. Adenine and thymine are also base pairs. The base pairing rule allows DNA to be replicated when cells divide, because the strands on one helix can have their base pairs made up to match.

 

Genes determine the phenotype (what the resulting organism will look like). There are often different forms of the same gene (each of which has, for instance, a different eye colour), and these are known as alleles. When a new organism is formed –sexually, it will often inherit two different alleles , one from each parent, but only one allele will be used, while the other remains dormant. We call one allele “dominant” and the other “recessive”. In human eyes, for instance, the allele for brown cornea is dominant to its blue counterpart, so if a person with brown eyes (BB) mates with a person with blue eyes (bb), their child will have brown eyes (Bb). However, if a Bb person mates with a bb person, at least some of their children will have blue eyes again.

As mentioned earlier, DNA molecules are coiled into structures called chromosomes. These consist of one double-stranded DNA molecule tightly coiled around proteins called histones. The number of chromosomes in a cell is different depending on the species. Humans normally have 46, of which there are 22 pairs common to both sexes. The 23rd pair, for females, consists of two X shaped chromosomes, but males have a mismatched pair of one X and one Y shaped. It is this difference which determines a person’s gender. Other species have different numbers of chromosomes: Goats have 60, cats have 38, kangaroos have 16, and snails have 24. The number of chromosomes that an organism has is believed to depend on the number of evolutionary stages which came before its own phenotype.

 

Occasionally, people can be born with the wrong number of chromosomes. People with Down Syndrome, for instance, have an extra 21st chromosome, and girls with Turner Syndrome are born missing one X chromosome. These birth defects often lead to slowed development, and physical deformities.

 

DNA as we now know it was first discovered in 1953 by Cambridge University scientists James Watson and Francis Crick. Up until that point, it had been accepted that DNA was the substance which carried genes from one generation to the next, but little was known about what DNA actually was, or how it worked.

 

Watson and Crick used the research done by Rosalind Franklin, in which she used X-ray diffraction to take pictures of crystalline DNA. Watson and Crick used this research to work out the double helix structure of DNA to match the photographs. Their proposal is now accepted by scientists worldwide. Watson and Crick were awarded the Nobel Prize in 1962, but by then Franklin had died of cancer, and the prize is not allowed to be awarded posthumously. 

 

Originally written January 2012 by Robin Taylor. Scored at A