Nucleic Acids and Cell Division

Nucleotide structure and nucleic acids

Individual nucleotides are made up of three components that combine by condensation reactions. These are:

• A phosphate group (H3PO4). This has the same structure in all nucleotides.

• A pentose sugar

• An organic base which contains nitrogen.

An organic base belongs to one of two different groups:

• The pyrimidine bases are thymine and cytosine

• The purine bases are adenine and guanine

The structure is DNA

• DNA is a double stranded polymer of nucleotides or polynucleotide.

• Each polynucleotide may contain many million nucleotide units.

• It is in the form of a double helix, the shape is maintained by hydrogen bonding.

• The pentose sugar is always deoxyribose.

• DNA contains four organic bases. These are adenine, guanine, cytosine and thymine.

• Each strand is linked to the other by pairs of organic bases.

• Cytosine always pairs with guanine, adenine always pairs with thymine, and the bases are joined hydrogen bonds. As a result of these pairings adenine is said to be  complementary of thymine and guanine is said to be complementary to cytosine.

  

How DNA is well suited to carry out its functions?

• It is a very stable molecule and can pass from generation to generation without change.

• It is a very large molecule and can carry a large amount of genetic information.

• The two strands are able to separate easily as they are held together by weak hydrogen bonds.

• As the base pairs are held within the deoxyribose-phosphate backbone, the genetic information is protected from outside forces.

The structure of RNA

• RNA is a single stranded polymer of nucleotide.

• RNA contains the pentose sugar, ribose.

• RNA contains the organic bases adenine, guanine, cytosine, and uracil (in place of thymine).

There are three types of RNA are involved in the process of protein synthesis:

• Messenger RNA (mRNA) is a long, single stranded molecule formed into a helix, it is manufactured in the nucleus and carries the genetic code from the DNA to the ribosomes in the cytoplasm.

• Ribosomal RNA (rRNA) is found in the cytoplasm and is a large, complex molecule made up of both double and single helices. Ribosomes are made up of ribosomal RNA and protein. It is the site of translation of genetic code.

• Transfer RNA (tRNA) is a small single stranded molecule. It forms a clover leaf shape, with one end if the chain ending in a cytosine-cytosine-adenine sequence at which point the amino acid it carries attaches itself. At the opposite end of the chain is a sequence of three bases called the anticodon. tRNA molecules transport amino acids to the ribosome so that proteins can be synthesised.

Comparison of DNA and RNA:

Sugar:

DNA= Deoxyribose RNA= Ribose

Bases:

DNA= CGAT RNA= CGAU

Helix:

DNA= Double RNA= Single

Scientists initially thought that proteins played a role in the transfer of hereditary material from one generation to the next. Although we now take for granted that DNA is the hereditary material…