Genomics

Lecture 6By Ms. Shumaila Azam

Gene Expression• Gene expression is the activation of a gene that results in a

protein.• The process by which a gene's information is converted into

the structures and functions of a cell by a process of producing a biologically functional molecule of either protein or RNA (gene product) is made.

• Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product.

• Gene expression takes place differently in prokaryotes and eukaryotes.

Steps

• Several steps in the gene expression process may be modulated, including the – transcription,– RNA splicing, – translation, and – post-translational modification of a protein.

Gene Expression

• The process of gene expression is used by all known life - eukaryotes (including multicellular organisms), prokaryotes (bacteria), possibly induced by viruses - to generate the macromolecular machinery for life.

Prokaryote Vs Eukaryotes

• Prokaryotes– No membrane bound

organelles (nucleus)– More primitive organisms– Only one circular

chromosome– Bacteria are the only

organisms that are prokaryotes.

• Eukaryotes– Membrane bound organelles

( specialize in function –nucleus, mitochondria, chloroplast)

– Chromosomes are in pairs and not circular

– All organisms that are not bacteria: protist, fungi, plants and animals

Euchromatin

• In Eukaryotes, following mitosis or meiosis, DNA recoils but certain regions remain relaxed for transcription. The areas of relaxed DNA are called euchromatin.

Gene Expression• DNA in eukaryotes has regions of coding and non

coding DNA. The regions of DNA that code for proteins or traits are called EXONS, while the regions that do not code for proteins are called INTRONS.

Gene Expression

• In prokayotes, transcription and translation occur in the cytoplasm.

• In eukaryotes, transcription occurs inside the nucleus in a two step sequence of events.– Pre-mRNA includes both introns and exons for the

gene.– mRNA is only the coding portion (exons).

• Translation occurs in the cytoplasm at the ribosomes.

Gene Expression

• In Prokaryotes there are three (3) regulatory elements that control gene expression.

1. Structural genes – genes that code for a specific polypeptide (protein).

2. Promoter – DNA segment that recognizes RNA polymerase.

3. Operator – element that serves as a binding site for an inhibitor protein that blocks transcription.

Enhancer Control• Eukaryote genes on a DNA

strand also have non coding control sequences that facilitate transcription.

• These are called enhancers.• Transcription factors are

additional proteins that bind to RNA polymerase and enhancers to help with transcription.

Regulation of gene expression

• Regulation of gene expression refers to the control of the amount and timing of appearance of the functional product of a gene.

• Example:– Control of insulin expression– X-chromosome inactivation– Cyclin expression

Types of genes depending upon how they are regulated

• Constitutive genegene that is transcribed continually.

• Facultative gene gene that is transcribed when needed.• Housekeeping gene

gene that is transcribed at a relatively constant level. They are required for the maintenance of cell.

• Inducible genegene whose expression is either responsive to the environmental change or dependent on the position in cell cycle.

• In genetics, gene expression is the most fundamental level at which the genotype gives rise to the phenotype.

Gene Regulation

• The genetic code stored in DNA is "interpreted" by gene expression, and the properties of the expression give rise to the organism's phenotype.

• Gene regulation gives the cell control over structure and function, and is the basis for cellular differentiation, morphogenesis and the versatility and adaptability of any organism.

Cell Differentiation

• Cell differentiation is the development of cells into cells with specialized functions.– Examples: muscle cells, liver cell, red blood cells

• As organisms grow and develop, organs and tissues develop to produce a characteristic form. The process is call morphogenesis.

E.g. Homeotic Genes

• Homeotic genes are regulatory genes that determine where certain anatomical structures, such as appendages, will develop in an organism during morphogenesis.

• These seem to be the master genes of development

Normal Mutant with legs growing out of head

Homeobox• In Drosophila (fruit flies) the

specific DNA sequence within a homeotic gene that regulates patterns of development is the homeobox.

• The same or very similar homeobox sequences have been found in many other eukaryotic organisms