Replication of DNAReplication of DNA

To be able to explain:To be able to explain:• the semi-conservative mechanism of DNA the semi-conservative mechanism of DNA

replication, including the role of DNA polymerase.replication, including the role of DNA polymerase.• How DNA acts as a genetic code by controlling the How DNA acts as a genetic code by controlling the

sequence of amino acids in a polypeptide.sequence of amino acids in a polypeptide.• That codons for amino acids are triplets of That codons for amino acids are triplets of

nucleotide bases.nucleotide bases.• Role of nucleic acids in protein and enzyme Role of nucleic acids in protein and enzyme

synthesissynthesis• The structure of RNA.The structure of RNA.

Function of DNA

• DNA is the genetic material, and genes are made of DNA is the genetic material, and genes are made of DNA. DNA.

• DNA therefore has two essential functions: replication DNA therefore has two essential functions: replication and expression.and expression.

• Replication means that the DNA, with all its genes, Replication means that the DNA, with all its genes, must be copied every time a cell divides.must be copied every time a cell divides.

• Expression means that the genes on DNA must control Expression means that the genes on DNA must control characteristics. characteristics.

• Characteristics are controlled by genes through the Characteristics are controlled by genes through the proteins they code for.proteins they code for.

Replication - DNA Synthesis

• DNA is copied, or replicated, before DNA is copied, or replicated, before every cell division.every cell division.

• Two new strands are built up by Two new strands are built up by complementary base-pairing onto the complementary base-pairing onto the two old strands.two old strands.

ReplicationReplication

• The enzyme helicase unwinds and unzips DNAThe enzyme helicase unwinds and unzips DNA

• The enzyme DNA polymerase joins the new The enzyme DNA polymerase joins the new nucleotides to each other nucleotides to each other

• DNA replication can takes a few hours. DNA replication can takes a few hours.

• Bacteria can reproduce so fast is that they have a Bacteria can reproduce so fast is that they have a relatively small amount of DNA. relatively small amount of DNA.

• In eukaryotes replication is speeded up by taking In eukaryotes replication is speeded up by taking place at thousands of sites along the DNA place at thousands of sites along the DNA simultaneouslysimultaneously

SEMICONSERVATIVE SEMICONSERVATIVE REPLICATIONREPLICATION

Original Original polynucleotide polynucleotide strandstrand

New New polynucleotide polynucleotide strandsstrands

Structure in relation to functionStructure in relation to function

• sequence of bases sequence of bases soso code for information code for information storagestorage

• long molecule long molecule soso large amount of large amount of information storedinformation stored

• complementary pairing complementary pairing soso information can information can be replicatedbe replicated

• double helix makes molecule stable double helix makes molecule stable soso prevents code being corruptedprevents code being corrupted

• chains held together by weak hydrogen chains held together by weak hydrogen bonds chains bonds chains soso chains can unzip for chains can unzip for replication / transcriptionreplication / transcription

RNA

• RNA is a nucleic acid like DNA, but with RNA is a nucleic acid like DNA, but with 4 differences:4 differences:– RNA has the sugar ribose instead of RNA has the sugar ribose instead of

deoxyribosedeoxyribose– RNA has the base uracil instead of RNA has the base uracil instead of

thyminethymine– RNA is usually single stranded, but can RNA is usually single stranded, but can

fold into 3-dimentional structures, like fold into 3-dimentional structures, like proteins.proteins.

– RNA is usually shorter than DNARNA is usually shorter than DNA

Messenger RNA (mRNA)

• mRNA carries the "message" that codes for a mRNA carries the "message" that codes for a particular protein from the nucleus (where particular protein from the nucleus (where the DNA master copy is) to the cytoplasm the DNA master copy is) to the cytoplasm (where proteins are synthesised). (where proteins are synthesised).

• It is single stranded and just long enough to It is single stranded and just long enough to contain one gene only. contain one gene only.

• It has a short lifetime and is degraded soon It has a short lifetime and is degraded soon after it is used.after it is used.

Ribosomal RNA (rRNA)

• rRNA, together with proteins, form rRNA, together with proteins, form ribosomesribosomes

• rRNA is coded for by numerous genes in rRNA is coded for by numerous genes in many different chromosomes. many different chromosomes.

• Ribosomes free in the cytoplasm make Ribosomes free in the cytoplasm make proteins for use in the cell.proteins for use in the cell.

• Ribosomes attached to the RER make Ribosomes attached to the RER make proteins for export.proteins for export.

Transfer RNA (tRNA)

• tRNA is an “adapter” that matches amino tRNA is an “adapter” that matches amino acids to their codon.acids to their codon.

• tRNA is only about 80 nucleotides long, and tRNA is only about 80 nucleotides long, and it folds up by complementary base pairing to it folds up by complementary base pairing to form a looped clover-leaf structure. form a looped clover-leaf structure.

• At one end of the molecule there is always the base At one end of the molecule there is always the base sequence ACC, where the amino acid binds. sequence ACC, where the amino acid binds.

• On the middle loop there is a triplet nucleotide On the middle loop there is a triplet nucleotide sequence called the anticodon. sequence called the anticodon.

• There are 64 different tRNA molecules, each with a There are 64 different tRNA molecules, each with a different anticodon sequence complementary to the different anticodon sequence complementary to the 64 different codons. 64 different codons.

• The amino acids are attached to their tRNA molecule The amino acids are attached to their tRNA molecule by specific aminoacyl tRNA synthase enzymes. by specific aminoacyl tRNA synthase enzymes.

Comparison of DNA and RNA

DNADNA RNARNA

SugarSugar

BasesBases

ShapeShape

Where foundWhere found

StabilityStability

PermanencePermanence

TypesTypes

ConcentrationConcentration

Molecular massMolecular mass

DeoxyriboseDeoxyribose RiboseRibose

A T C GA T C G A U C GA U C GDouble helixDouble helix Single strandSingle strand

In nucleusIn nucleusNucleus and Nucleus and

cytoplasmcytoplasmVery stableVery stable Less stableLess stablePermanentPermanent TemporaryTemporary

OneOne Three – m t and rThree – m t and rConstantConstant VariableVariableV largeV large smallersmaller

How structure is related to functionDNADNA

• FunctionFunction 1. To pass on hereditary information1. To pass on hereditary information

2. To control production of proteins2. To control production of proteins

StructureStructure FF How structure is relatedHow structure is related

Double helix is Double helix is stablestable

Bases act as codeBases act as code

4 bases4 bases

2 strands 2 strands connected by connected by hydrogen bondshydrogen bonds

Helix encloses Helix encloses hydrogen bondshydrogen bonds

Long, large Long, large moleculemolecule

Folds into coilFolds into coil

11 Maintains information storeMaintains information store

22 Order acts as instructionOrder acts as instruction

22 Allow for a triplet codeAllow for a triplet code

1&21&2 Allows unzipping for copyingAllows unzipping for copying

11 StabilityStability

1&21&2 Holds a lot of information togetherHolds a lot of information together

11 Compact shapeCompact shape

m RNAm RNA FunctionFunction 1. Translation1. Translation

StructureStructure FunctionFunction How structure is relatedHow structure is related

Single strandSingle strand

Bases as Bases as codoncodon

Can pass through nuclear Can pass through nuclear porepore

Act as code for amino Act as code for amino acidsacids

tRNA – tRNA – pick up their specific amino acids pick up their specific amino acids

from the cytoplasmfrom the cytoplasm

Transcription - RNA Synthesis

• DNA never leaves the nucleus, but proteins DNA never leaves the nucleus, but proteins are synthesised in the cytoplasmare synthesised in the cytoplasm

• A copy of each gene is made to carry the A copy of each gene is made to carry the “message” from the nucleus to the “message” from the nucleus to the cytoplasm. cytoplasm.

• This copy is mRNA, and the process of This copy is mRNA, and the process of copying is called transcription.copying is called transcription.

t RNAt RNA FunctionFunction 1. Amino acid 1. Amino acid activationactivation

StructureStructure FunctionFunction How structure is relatedHow structure is related

20 forms20 forms

AnticodonAnticodon

For 20 amino acidsFor 20 amino acids

To pair with codon on To pair with codon on m RNAm RNA

• Only one strand of RNA is made. Only one strand of RNA is made.

• The DNA stand that is copied is called the The DNA stand that is copied is called the template or sense strand because it contains template or sense strand because it contains the sequence of bases that codes for a the sequence of bases that codes for a protein. protein.

• The other strand is just a complementary The other strand is just a complementary copy, and is called the non-template or copy, and is called the non-template or antisense strand.antisense strand.

• The new nucleotides are joined to each The new nucleotides are joined to each other by strong covalent other by strong covalent phosphodiester bonds by the enzyme phosphodiester bonds by the enzyme RNA polymerase.RNA polymerase.

• The introns are cut out and the exons are The introns are cut out and the exons are spliced together by enzymes. spliced together by enzymes.

• The result is a shorter mature RNA The result is a shorter mature RNA containing only exons. containing only exons.

• The introns are broken down.The introns are broken down.• The mRNA diffuses out of the nucleus The mRNA diffuses out of the nucleus

through a nuclear pore into the cytoplasm.through a nuclear pore into the cytoplasm.

Translation - Protein Synthesis

• A ribosome attaches to the mRNA at an A ribosome attaches to the mRNA at an initiation codon (AUG).initiation codon (AUG).

• A codon is a sequence of three A codon is a sequence of three nucleotidesnucleotides

• The ribosome encloses two codons.The ribosome encloses two codons.

1.

•First the mRNA attaches itself to a ribosome (to the small subunit).First the mRNA attaches itself to a ribosome (to the small subunit).•Six bases of the mRNA are exposed.Six bases of the mRNA are exposed.•A complementary tRNA molecule with its attached amino acid A complementary tRNA molecule with its attached amino acid (methionine) base pairs via its anticodon UAC with the AUG on the (methionine) base pairs via its anticodon UAC with the AUG on the mRNA in the first position P.mRNA in the first position P.•Another tRNA base pairs with the other three mRNA bases in the Another tRNA base pairs with the other three mRNA bases in the ribosome at position A.ribosome at position A.•The enzyme peptidyl transferase forms a peptide bond between the two The enzyme peptidyl transferase forms a peptide bond between the two amino acids.amino acids.•The first tRNA (without its amino acid) leaves the ribosome.The first tRNA (without its amino acid) leaves the ribosome.

Translation 2

The ribosome moves along the mRNA to the next codon (three The ribosome moves along the mRNA to the next codon (three bases).bases).

The second tRNA molecule moves into position P.The second tRNA molecule moves into position P.

Another tRNA molecule pairs with the mRNA in position A bringing Another tRNA molecule pairs with the mRNA in position A bringing its amino acid.its amino acid.

A growing polypeptide is formed in this way until a stop codon is A growing polypeptide is formed in this way until a stop codon is reached.reached.

End of Translation

A stop codon on the mRNA is reached and this signals the ribosome A stop codon on the mRNA is reached and this signals the ribosome to leave the mRNA. A newly synthesised protein is now complete!to leave the mRNA. A newly synthesised protein is now complete!

Translation - animationTranslation - animation