THE MOLECULAR GENETICS OF GENE

EXPRESSION

By: Christone Jay P. NovidaAAPD2B

Gene expression

Gene expression

is the process by which information from a gene is used in the synthesis of a functional gene product

These products are often proteins, but in non-protein coding genes such as ribosomal RNA (rRNA), transfer RNA (tRNA) or small nuclear RNA (snRNA) genes

The process of gene expression is used by all known life:

Eukaryotes-including multicellular organisms.

Prokaryotes -bacteria and archaea.  Viruses - to generate the

macromolecular machinery for life.

Steps of gene expression

 transcription RNA splicing translation  post-translational modifacation of a

protein.

Gene transcription

Gene transcription

The process by which genetic information is copied from DNA to RNA, resulting in a specific protein formation.

5 steps of gene transcription

Pre-initiation Initiation Promoter clearance Elongation Termination

Pre-initiation contains:

 1. Core Promoter Sequence 2. Transcription Factors 3. RNA Polymerase 4. Activators and Repressors

Inititation

At the start of initiation, the core enzyme is associated with a sigma factor that aids in finding the appropriate -35 and -10 base pairs downstream of promoter sequences. When the sigma factor and RNA polymerase combine, they form a holoenzyme.

Promoter clearance

After the first bond is synthesized, the RNA polymerase must clear the promoter. During this time there is a tendency to release the RNA transcript and produce truncated transcripts. This is called abortive initiation and is common for both eukaryotes and prokaryotes. Abortive initiation continues to occur until the σ factor rearranges, resulting in the transcription elongation complex (which gives a 35 bp moving footprint).

Elongation

One strand of the DNA, the template strand (or noncoding strand), is used as a template for RNA synthesis. As transcription proceeds, RNA polymerase traverses the template strand and uses base pairing complementarity with the DNA template to create an RNA copy

Termination

Transcription termination in eukaryotes is less understood but involves cleavage of the new transcript followed by template-independent addition of As at its new 3' end, in a process called polyadenylation

Gene translation

Translation

translation is the third stage of protein biosynthesis(part of the overall process of gene expression).

In translation, messenger RNA (mRNA) produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein

3 phases of Translation

1.  activation -  the correct amino acid is covalently bonded to the correct transfer RNA (tRNA). The amino acid is joined by its carboxyl group to the 3' OH of the tRNA by an ester bond. 

2.  Initiation- involves the small subunit of the ribosome binding to the 5' end of mRNA with the help of initiation factors(IF)

3. Termination of the polypeptide happens when the A site of the ribosome faces a stop codon (UAA, UAG, or UGA)

Genetic code

Genetic code

The genetic code is the set of rules by which information encoded in genetic material (DNA or mRNA sequences) is translated into proteins(amino acid sequences) by living cells

Codon

a unit of three adjacent nucleotides along a DNA or messenger RNA molecule that designates a specific amino acid to be incorporated into a polypeptide. The order of the codons along the DNA or messenger RNA determines the sequence of the amino acids in the polypeptide

RNA codon table