Nucleic Acids, DNA Replication and Protein Synthesis.

Protein synthesis is a continual process. It can be separated into two main aspects, which are:

• The events in the nucleus that involve the transcription of DNA message onto the mRNA molecule.
• The message that is translated into polypeptide molecules and hence into proteins in the cytoplasm.

In the DNA of the chromosomes, there are large amounts of information stored. To make the human body, each cell has a full complement of chromosomes with all genetic information. Protein synthesis occurs on a ribosome, which are found on the rough endoplasmic reticulum in the cytoplasm. This is a complex of ribosomal RNA and proteins. It is made up of two subunits:

• Large – three rRNA and about 49 proteins
• Small – one rRNA and about 33 proteins

2) DNA is the basic hereditary material in all cells and contains all the information necessary to make proteins.  DNA and RNA, like proteins and polysaccharides, are macromolecules. They are made up of many similar, smaller molecules joined into a long chain that makes them polymers. Nucleotides are smaller molecules from which DNA and RNA are made up.   Therefore DNA and RNA are polynucleotides, and are just referred to as nucleic acids. Each nucleotide has three parts – a five-carbon or pentose sugar, a nitrogen-containing base and a phosphate group.

The pentose sugars

        The sugar is one of the very two similar pentose rings. Ribonucleic acids contain the sugar ribose. Deoxyribonucleic acids contain the sugar deoxyribose. The only difference between these sugars is tat deoxyribose contains one oxygen atom less than ribose.

Transfer RNA (tRNA)

This is found in the cytoplasm. Its function is to pick up particular amino acids, which are always free and carries them to the ribosome where they are all joined to form a protein. Each amino acid has its own specific tRNA molecule. It is involved in the process of protein synthesis.

 

Messenger RNA (mRNA)

This is formed in the nucleus in a similar way to the replication of DNA. A part of the DNA double helix ‘unzips’ and the RNA nucleotides pair up with those on one of the DNA strands. They are joined to form a strand of mRNA. A piece of mRNA usually carries instructions for just one polypeptide. It is formed on the 5’ strand of the DNA. It is involved in the process of protein synthesis.

DNA is involved in the synthesis of proteins. The human body manufactures 20 amino acids, which together with the essential amino acids obtained through diet are synthesised to form proteins. DNA controls the chemical reactions in the cells which are known to be activated by enzymes, Enzymes are proteins and DNA is a code for exactly which polypeptides and proteins are manufactured. DNA controls protein structure by determining the exact order in which the amino acids join together when proteins are synthesised. The interactions between the R groups of amino acids that determine the protein's 3D structure, DNA controls the exact order of the amino acids.

Hydrogen bonds between the bases break in the nucleus as part of the DNA molecules unwind and unzip. Any free RNA nucleotides pair up with the exposed bases of one strand only.

        Sugar-phosphate groups are bonded together to form a sugar-phosphate backbone while RNA nucleotides pair themselves up with their complementary ones. This forms a single stranded molecule, which is called messenger RNA (Mrna). This then leaves the nucleus through a pore in the nuclear envelope.

 

In the cytoplasm, there are free transfer RNA (Trna) and amino acids. Of each Trna molecule, an amino acid can bind to it of one end. On the other end of the Trna molecule, there are three unpaired bases, which are known as anticodon. This takes place under the control of a specific enzyme and with energy from ATP.

Also in the cytoplasm, the Mrna molecule attaches to a ribosome. Ribosomes consist of ribosomal RNA and protein and contain a small and large subunit. These two subunits combine to form polysome. The Mrna binds to the small subunit.

AUG is always the first three exposed bases, or codon. A Trna molecule with the complementary anticodon, UAC, forms hydrogen bonds are formed with this codon. This molecule has the amino acid methionine attached to it.

With the next three exposed bases, a second Trna molecule bonds with them. This brings another amino acid. The two amino acids are held together closely which forms a peptide bond between them. The reaction is catalysed by an enzyme, which is found in the small subunit of the ribosome.

The ribosome moves along the Mrna, ‘reading’ the next three bases on the ribosome. A third Trna molecule brings a third amino acid, which joins to the second one. The first Trna then leaves.

The polypeptide chain continues to extend, until a ‘stop’ codon is exposed on the ribosome, which is UAA, UAC or UGA.