Gene Function: Genes in Action

Chapter 11Key Knowledge:molecular genetics principal events in transcription and translation; cell reproduction: cell cycle, DNA replication, apoptosis; binary fission; gene regulationprokaryote chromosome and plasmids

Gene ActionGene action involves two processes:

Transcription (t/c)Translation (t/l)

After the next few slides we will be able to answer the following questions:How do we get from the bases found in DNA to

amino acids? How do we get from a bunch of amino acids to

proteins?

RNA versus DNADNA

Usually double strandedDeoxyribose sugar Bases: C,G A,T

RNAUsually single strandedRibose sugarBases: C,G,A,U

Both contain a sugar, phosphate, and base.

The Central Dogma:Transcription and Translation

Transcription

Translation

DNA

RNA

Protein

TranscriptionHas a number of key processes:

mRNA is made using DNAmRNA is then modified

During transcription, a DNA sequence is read by RNA polymerase, which produces a complementary, antiparallel mRNA strand.

DNA forms base pairs with RNA:

DNA RNAC GG CT AA U

RNA PolymeraseRNA polymerase is an

enzyme complex that catalyses the transcription of DNA to RNA.

It begins transcription by binding to the DNA double helix and gradually unwinds the helix, exposing the DNA template to be transcribed.

Transcription VideoVideo

After TranscriptionAfter transcription occurs you are left with an

mRNA strand.However, this strand needs to be modified

further in order to be translated.This is called post-transcriptional modification.During this process, the introns in the mRNA

strand are removed, and the final mRNA molecule is chemically caped and a poly-A tail is attached.

TranslationInvolves three types of RNA: mRNA, tRNA

and rRNATranslation involves the use of mRNA (from

transcription) held in a ribosome by rRNA (ribosomal RNA) to make a protein.

Amino acids are transferred by the tRNA (transfer RNA) to their matching codons, and hence a protein is assembled.

TranslationTranslation has three main stages:

InitiationElongationTermination

All three stages occur on the ribosomesRibosomes contain 3 binding sites

E - TerminationP - elongationA - initiation

TranslationtRNA brings amino acids to the ribosome

tRNATransfer RNAamino acid bound

to one endAnticodon on the

other end complements mRNA codon

tRNA FunctionAmino acids must be in the correct order for

the protein to function correctly.tRNA lines up amino acids using mRNA code

Reading the DNA codeEvery DNA base pairs with a mRNA baseCodon - coding triplet of DNA bases and

corresponding mRNA basesEvery group of 3 mRNA bases encodes for a

single amino acid

Translation VideoVideo

Reading the mRNA code

Reading the mRNA code

Synthesising new DNADNA needs to be copied exactly so that it can

be passed from one cell to another. This process is called DNA replication.

During DNA replication each strand of parental DNA molecule acts as a template strand on which a new strand is synthesised.

DNA ReplicationDNA replication begins when a region of

double-stranded DNA unwinds to form a region of single-stranded DNA.

Single nucleotide are the raw materials for the process of DNA replication.

The nucleotides come into place – where there is a G in the template strand, a C-containing nucleotide is brought into place.

In this way, the base sequence of the double-strand of DNA controls the order of the nucleotides in two new single strands of DNA.

This process is catalysed by the enzyme DNA polymerase.

Each nucleotide joins to its neighbour in the chain by a strong sugar-phosphate bond.

The strand is always copied from 5’ to 3’.

Which model of DNA replication?

How do we know which model is correct?

The mystery of the model of DNA replication was solved in 1958 by Meselsohn and Stahl.They took bacterial cells and allowed them to

multiply for several generations in a growth medium containing ‘heavy’ nitrogen (15N) only, so that all nitrogen atoms in these bacterial DNA were heavy nitrogen.

These bacterial cells were then transferred to a new growth medium containing only normal nitrogen (14N).

After one generation, a sample of the new bacterial cells was collected. Any new DNA made by this new generation could only include 14N atoms.

The DNA was extracted and the relative weight of the DNA was determined using a centrifuge.

The lower down the tube, the heavier the DNA.

They obtained a single band with their experiment. Showing that replicated DNA contained one original chain and one new chain. This means that DNA replication is semi-conservative

apoptosis; binary fission; gene regulation, prokaryote chromosome.