Gene expression
Presented by
Mr.Karthick J

Gene Expression
• Gene expression is the process by which the genetic code – the
nucleotide sequence – of a gene is used in the synthesis of a functional
gene product.
• It refers to a complex series of processes in which the information
encoded in a gene is used to produce a functional product such as a
protein that dictates cell function.
• It involves several different steps through which DNA is converted to an
RNA which in turn is converted into a protein or in some cases RNA, for
example, genes encoding the necessary information for transfer RNAs
and ribosomal RNAs (tRNAs and rRNAs).

• The two main stages include:
• Transcription: the production of messenger RNA (mRNA) by
the enzyme RNA polymerase, and the processing of the
resulting mRNA molecule.
• Translation: the use of mRNA to direct protein synthesis,
and the subsequent post-translational processing of the
protein molecule.
• Thus, gene expression is the phenotypic manifestation of a
gene or genes by the processes of genetic transcription and
genetic translation.

Sequence of Events in Gene Expression
• When genes are expressed, the genetic information (base
sequence) on DNA is first copied to a molecule of mRNA
(transcription).
• The mRNA molecules then leave the cell nucleus and enter the
cytoplasm (in eukaryotes), where they participate in protein
synthesis by specifying the particular amino acids that make up
individual proteins (translation).

phases in gene expression
• Gene expression consists of steps that finally produce a functional bio-
molecule.
• steps involved in gene expression include the following.
• Transcription – conversion of DNA to RNA
• This is the first step in gene expression in which DNA molecules are
transcribed into their corresponding RNA copy.This process is aided by
an enzyme called DNA-dependent RNA polymerase.
• Post-transcriptional modifications
• In this process, the primary RNA obtained after transcription is modified
to produce a mature messenger RNA or mRNA.

The processes involved are
• Splicing which is the cleavage of introns (non-coding sequences) and
ligation of exons (coding sequences) with the help of several
components that recognize specific sequences in the RNA.
• Capping which involves addition of a cap molecule to the 5’ end.
• Tailing which is the addition of poly A tail to the 3’ end.
• RNA transport (In Eukaryotes)
• Most of the mature mRNAs produced after modifications are
transported from the nucleus to the cytoplasm where the next step
in gene expression takes place.
• This is achieved by moving the mRNAs through tiny pores in the
nucleus to reach the cytosol.

Translation or protein synthesis
• The sequence in the mRNA is translated into a protein with the help of
several components such as ribosomes, tRNAs or transfer RNAs, and
enzymes called aminoacyl tRNA synthetases.
• Translation of mRNA involves 3 important steps – initiation, elongation,
and termination, leading to the formation of polypeptide chains.
• Protein folding and modifications
• In this final step, the polypeptide chains or random coils formed during
translation fold into a 3D structure giving rise to a functional protein.
• Failure to fold leads to protein inactivity and misfolded proteins have
abnormal functionalities compared to correctly folded ones.
• Also, proteins can be modified by various methods such as
phosphorylation, glycosylation, ADP ribosylation, hydroxylation, and
addition of other groups.

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