U3 AOS1 Topic 4: Genes, Proteins & Exocytosis

 GENES

Gene is a part of DNA that contains code to make a specific protein. Genetic code refers to instructions present in a gene that tell a cell how to make a specific protein. For example, alpha haemoglobin is a protein that is coded by a gene HBA1. The information stored in DNA as genes is used by cells to synthesize proteins. This process is called as gene expression. Gene expression involves two processes.

1.      DNA is copied into mRNA (messenger RNA) by the process of transcription.

2.       mRNA sequence is read to produce a sequence of amino acids(polypeptide) by the process of translation.

    Transcription occurs inside the nucleus where DNA is present. Once the mRNA is formed, it moves out of the nucleus through nuclear pores toward ribosomes in cytoplasm where translation can take place. Ribosomes read the sequence of codons in mRNA and incorporate specific amino acids to make a chain of amino acids in exact sequence following the instructions of genetic code.

The codon table:

The codon table is used to determine amino acid coded for by a specific codon. Genetic code is a set of codons. Each codon is a sequence of three nucleotides. There are 4 types of nucleotides in a DNA molecule i.e. Adenine, Guanine, Cytosine, and Thymine represented by letters A, G, C, and T respectively. These 4 nucleotides can make 64 different combinations of 3 nucleotides each.

 A codon corresponds to a specific amino acid or a stop codon. For example, AUG codes for amino acid methionine and UGA is a stop codon. There are a total of 20 amino acids. An amino acid can be coded by more than one codon (degenerate). The genetic code in DNA refers to the instructions for the synthesis of proteins in a cell.

Translation begins with start codon where amino acid methionine is incorporated. Translation continues until the stop codon reaches. Stop codon include UAA, UAG, and UGA. These codons do not code for any amino acid and therefore stop the translation process. The correct sequence of amino acids obtained after the completion of translation is called the primary structure of proteins. 

Properties of Genetic Code:

1.      Universal: All living organisms have the same codons to code for specific amino acids.

2.      Unambiguous: Each codon can only code for a specific amino acid.  

3.      Degenerate: Each amino acid may be coded for by more than one codon.

4.      Non-overlapping: Each triplet codon is read independently. For example, AUU is codon 1 and CGA is codon 2.

Gene Structure:

Genes can be composed of many different components including:

1.      Promoter region: The promoter region is a sequence of DNA where RNA polymerase enzyme binds. RNA polymerase enzyme is responsible for transcription of DNA to mRNA. The promoter region is located upstream of a gene. It means that it is located towards the 5' end of DNA strand which is to be copied. Therefore, the promoter region indicates the starting position and direction of transcription (found in both prokaryotes and eukaryotes).

2.      Introns: Introns are regions of non-coding DNA that do not code for any amino acid in the final protein as they are removed during RNA processing (found only in eukaryotes).

3.     Exons: Exons are regions of coding DNA, which are transcribed and translated into the final protein (found in both prokaryotes and eukaryotes)

4.      The termination sequence is a sequence of DNA that signals for the termination of transcription (found in both prokaryotes and eukaryotes).

5.      Operator The operator region serves as the binding site for repressor proteins. Repressor proteins can inhibit gene expression (found only in prokaryotes).

6.      Leader The leader region is the sequence of DNA involved in regulation of gene expression located upstream of the coding region but downstream of the promoter and operator (found in prokaryotes).

Exocytosis

Proteins once synthesized are folded and modified in the cell. Many proteins have to be exported outside the cell so that they can be used elsewhere in body. For this purpose proteins are packaged inside the vesicles and transported outside by the process of exocytosis.

Exocytosis is the process in which secretary vesicles fuse with the cell membrane to export the contents outside the cell. The phospholipid membrane of secretary vesicle can easily fuse with the phospholipid plasma membrane because of fluidity of membrane. Exocytosis includes following steps:

1.      Secretory vesicle is transported to the plasma membrane

2.      Membrane of secretory vesicle fuses with plasma membrane

3.      Secretory products are released outside the cell

 Secretory vesicles contain large substances including proteins which have to be secreted outside the cell. Apart from proteins and other large substances, waste products are also exported outside the cell by means of exocytosis. Exocytosis is a form of active transport therefore; energy is required for this purpose.

Protein Secretory Pathway:

Proteins are synthesized in ribosomes. Ribosomes can either be attached to the rough endoplasmic reticulum(RER) or freely floating in the cytoplasm. The polypeptide chain which is synthesized in ribosome by means of translation is correctly folded into protein and transported to golgi apparatus to be processed and packaged. The packaged proteins are then exported outside the cell by the process of exocytosis.