Microbial genetics

3.1 An overview of genetic possesses3.2 The basis of hereditary3.3 DNA replication3.4 RNA and protein synthesis3.5 Gene expression

Lecture content

Genetics – science of hereditaryGenome – the genetic information of a cellChromosomes – structures containing DNA

that physically carry hereditary informationGenes – segment of DNA (except some

viruses contain RNA)Base pair – consist of A(adenine), G

(Guanine), T (thyamine) and C (cytosine)

Terminologies

3.1 An overview of genetic posesses

Crossing over

3.2 The basis of hereditary

Nucleic acid in information storage

Types of bases

DNA replication

Process where one parental of double stranded DNA will split into 2 daughters dsDNA

The copies are exactly the same and doesn’t involve in protein production

There will be free-nuclotides that available surround the cells that permits the replication process

At this time, the new stands will copy it oppositely as pairing the bases in parental DNA, A with T and C with G, from 5’ to 3’, will be copied as 3’ to 5’

The process where induce by DNA polymeraseThe point at which the replication occur are called

as replication fork

DNA replication

DNA replication (Eukaryote)

The new daughter DNA will have one old strand and one newly replicate DNA and this is called as semiconservative replication

In special case like in some bacteria, E. coli, the process is bidirectionally around the cromosome.

There will be 2 forks move oppositely away from the origin of replication

DNA replication (Prokaryote)

RNA and protein synthesisTranscription and translation

Synthesis of a complementary strand of RNA from DNA template

As your know, there are three RNA in bacterial cell: messenger RNA, ribosomal RNA and transfer RNA

Ribosomal RNA (rRNA) – integral component in ribosome producing protein

Messenger RNA (mRNA) – carries the coded information for making specific proteins from DNA to ribosomes

Transcription

Overal process of RNA and protein synthesis

In transcription, the mRNA strands will copy the specific strands in DNA template

The component of nucleic acid bases will be produce in the mRNA pairing the bases in DNA template, for example, a G with C, via versa

However in RNA, there are no T so they replace it with U that will be paired with A

the DNA strand have 3’ and 5’ ends, so mRNA strand will start copy the bases oppositely, example if the DNA strand start from 3’ and end with 5’, 3’ – ATGCCTA – 5’, the mRNA will start copy and producing 5’ and end up with 3’, 5’ – UACGGAU - 3’

Transcription process

Transcription

The transcription process can only occur with present of RNA polymerase enzyme and supply of RNA nucleotides

The process begins when RNA polymerase binds to the DNA at site called promoter

The transcription continues until the RNA polymerase reaching the site in DNA called terminator

mRNA will become a intermediate storage of DNA information before the translation process take place

Traffic light of transcription

Translating the ‘language of nucleic acid’ to the ‘language of protein’

‘language’ of mRNA is in the form of codons, 3 nuclotides consider as 1 codon and coded for 1 specific protein, eg. UAG, GCC, UGG

There are 64 possiblity of codon but only 20 types of amino acid are synthesize

It is due to the degeneracy situation where some protein coded by more than 1 codon

Translation process

Sense codon – code for amino acidNonsense codon – also called as stop codon,

will stop the translation process

Types of codon

Translation

Transfer RNA (tRNA) – will recognize the specific codon and transport the particular amino acid

tRNA has an anticodon which are used to read the codon in the mRNA strands

Most probably the sequence in tRNA now are similar with the origin strand (DNA template)

Basically in the DNA genes itself compose of exon (seq that expressed) and introns (seq that do no encode protein)

All the introns will be removed by small nuclear ribonucleoproteins ( snRNPs) and combine all the exons together

Reasons behind translation

Genetic control mechanismsRepression

Induction

The Operon Model of Gene Expression

Most of microbial metabolic reaction need enzymes

Some enzymes are needed in a big amount through out the bacterial life as a living demands, for example the glucose product (enzymes of glycolysis)

In other cases, the enzyme where only needed in a particular amount and that is why the operon system present

Regulation of bacterial gene expression

Repression Inhibits gene expression and decreases the

synthesis of enzymesPrevent from overbundance of an end

product of metabolic pathwayThe protein use to decrease the rate is known

as repressorIt has ability to block RNA polymeraseThe default position of repressible gene is on

Induction Turns on the transcription of a gene The substance involve known as inducerThe enzyme that are synthesized in the

present of inducer are inducible enzymesEg. Enzyme β-galactosidase that split lactose

to glucose and galactose for E.coli

The Operon Model of gene expressionIntroduce by Francois Jacob and Jacques

Monod in 1961To account the regulation of protein

expressionGene that determined the surface of protein

is known as structural genesIn lac operon, there are two short DNA

segment known as promoter and operator

Terminologies Promoter – region of DNA where RNA

polymerase initiate transcriptionOperator – as a traffic light that instruct the

structural genes to be transcribedOperon – consist of operator, promoter and

three structural genes

System of the Operon

Operon on and off

The end