GENE EXPRESSION AND REGULATION Molecular Genetics GENES Genes

GENE EXPRESSION AND REGULATION Molecular Genetics

GENES • Genes are small sections of DNA within the genome that code for proteins. They contain the instructions for our individual characteristics – like eye and hair colour. • A typical gene consists of an upstream region (promoter region), coding segments (exons) and non-coding segments (introns)

LEARNING FOCUS Gene expression: the genetic code and roles of RNA in transcription, RNA processing in eukaryotes, and translation • Gene expression is the process by which the instructions in our DNA are converted into a functional product, such as a protein. • Gene expression is a tightly regulated process that allows a cell to respond to its changing environment. • It acts as both an on/off switch to control when proteins are made and also a volume control that increases or decreases the amount of proteins made. • There are two key steps involved in making a protein, transcription and translation.

TRANSCRIPTION and TRANSLATION Link to interactive http: //learn. genetics. utah. edu/content/basics/geneanatomy/

GENE EXPRESSION • The overall process of gene expression involves two major stages • The first stage of gene expression involves transcription, which is 'rewriting' or copying of information from DNA to ribonucleic acid (RNA). • The synthesised RNA is complementary to the sequence of one strand of the DNA (the template strand). The other strand is called the coding strand. • An important RNA molecule produced during transcription is called messenger RNA (m. RNA) because the molecule functions as a messenger, carrying a copy of the code into the cytoplasm.

Synthesising messenger RNA • Messenger RNA synthesis is controlled by the enzyme RNA polymerase. • A typical gene consists of an upstream region (promoter region), coding segments (exons) and non-coding segments (introns) • The promoter region of a gene has a specific sequence recognised by RNA polymerase that initiates transcription.

Synthesising m. RNA continued • The coded RNA (primary transcript) is modified by enzymes that cut out the regions that corresponded to the introns (non-coding DNA) of the gene and join the remaining pieces back together. • This shortened RNA molecule corresponding to the exon (coding) regions of the gene is the messenger RNA (m. RNA). The m. RNA molecule is complete when its two ends are modified. It is chemically 'capped' at the 5′ end. A tail of As (a poly-A tail) is added at the 3′ end. • This is the m. RNA that then moves out through the nuclear pores into the cytoplasm.

TRANSLATION – assembling proteins • The second stage of gene expression is translation. • In this process, the 'instructions' in the m. RNA are read and a polypeptide (protein) product is assembled. • The m. RNA is transported to the ribosomes that are either free in the cytoplasm or located on the rough endoplasmic reticulum. • The ribosomes provide a scaffold for the m. RNA to assemble. At the ribosomes, the sequence of bases in the m. RNA is 'decoded' to give the sequence of amino acids of the polypeptide.

GENE REGULATION • Gene regulation involves processes that control gene expression, turning a particular gene 'on' or 'off'. Your body only transcribes genes and produces proteins when they are needed in order to save energy and resources. • All the somatic cells in your body contain the same chromosomes and therefore the same DNA and same genes. • However, these cells are able to have different shapes and sizes and perform different functions and change throughout your lifespan. • These differences are possible because of different mechanisms that control the expression of individual genes. These mechanisms are collectively referred to as mechanisms for gene regulation.

GENE REGULATION • There are many steps involved in the expression of gene, therefore there are many different mechanisms for regulating expression: • • • The structure of genes varies The rate of transcription can be regulated Post-transcriptional modifications can influence which protein is produced The rate of translation can be regulated The activity of the protein product (enzyme) can be regulated • Genes can also be regulated by their environment • Light, Temperature, Ions, Hormones

GENE REGULATION • Gene regulation is not as well understood in multicellular organisms such as plants and mammals as it is in bacteria, but similar processes do apply. • To understand this we are going to look at how the lactose gene is turned on and off in E Coli.

lac operon • An operon is a group of genes that are all transcribed at the same time. • The lac operon consists of three genes each involved in processing the sugar lactose. • One of them is the gene for the enzyme β-galactosidase • This enzyme hydrolyses lactose into glucose and galactose

Bacteria adapting to its environment • E. coli can use either glucose, which is a monosaccharide, or lactose, which is a disaccharide for energy. • However, to use lactose it needs to be hydrolysed (digested) first. • So the bacterium prefers to use glucose when it can.

Four situations are possible 1. When glucose is present and lactose is absent the E. coli does not produce β-galactosidase. 2. When glucose is present and lactose is present the E. coli does not produce β-galactosidase. 3. When glucose is absent and lactose is absent the E. coli does not produce β-galactosidase. 4. When glucose is absent and lactose is present the E. coli does produce β-galactosidase

When lactose is absent • A repressor protein is continuously synthesised. It sits on a sequence of DNA just in front of the lac operon, the Operator site • The repressor protein blocks the Promoter site where the RNA polymerase settles before it starts transcribing Repressor protein DNA I O Regulator gene Operator site RNA polymerase Blocked z y lac operon a

When lactose is present • A small amount of a sugar allolactose is formed within the bacterial cell. This fits onto the repressor protein at another active site (allosteric site) • This causes the repressor protein to change its shape (a conformational change). It can no longer sit on the operator site. RNA polymerase can now reach its promoter site DNA I O z y a DNA I O z y Promotor site a

When both glucose and lactose are present • This explains how the lac operon is transcribed only when lactose is present. • BUT…. . this does not explain why the operon is not transcribed when both glucose and lactose are present. • When glucose and lactose are present RNA polymerase can sit on the promoter site but it is unstable and it keeps falling off Repressor protein removed RNA polymerase DNA I O z y Promotor site a

When glucose is absent and lactose is present • Another protein is needed, an activator protein. This stabilises RNA polymerase. • The activator protein only works when glucose is absent • In this way E. coli only makes enzymes to metabolise other sugars in the absence of glucose Activator protein steadies the RNA polymerase Transcription DNA I O z y Promotor site a

Summary Carbohydrates Activator protein Repressor protein RNA polymerase lac Operon + GLUCOSE + LACTOSE Not bound to DNA Lifted off operator site Keeps falling off promoter site No transcription + GLUCOSE - LACTOSE Not bound to DNA Bound to operator site Blocked by the repressor No transcription - GLUCOSE - LACTOSE Bound to DNA Bound to operator site Blocked by the repressor No transcription - GLUCOSE + LACTOSE Bound to DNA Lifted off Sits on the operator site promoter site Transcription