BIOL2146 DNA Replication and Gene Expression

8/3/2019 BIOL2146 DNA Replication and Gene Expression

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DNA replication

• Model for DNAreplication

• DNA strands areantiparallel

• DNA synthesis

– Leading strand

– Lagging strand

• Reference

– Campbell: Chap. 16



Model for DNA replication:

> semiconservative



Each DNA strand acts as a template

for a new strand• The 2 strands are complimentary

– Each stores the information necessary to

reconstruct the other strand – Due to base pairing: G – C and A – T

• Replicated (copied) DNA has: – One original strand

– One new strand



DNA Replication:

> the basic concept



Replication is bidirectional:

> DNA synthesis proceeds in both direction



“DNA polymerase” adds nucleotides



The two strands of DNAare antiparallel

DNA synthesis occurs inthe 5’ 3’ direction



DNA Replication:

> the problem of antiparallel DNA strands• DNA synthesis is in 5’ 3’ direction

• DNA synthesis of one strand is continuous

• Synthesis of other strand is discontinuous

– Enzyme must work away from replication fork

– Synthesized in short DNA fragments



The synthesis of one strand is

discontinuous

> lagging strand



Lagging strand

• short segments of newly synthesisedDNA

• Okazaki fragments

– Synthesis occurs in a backward direction

– 100-200 nucleotides long

• DNA ligase joins the fragments



Enzymes involved in DNA replication



Summary of DNA replication



Transcription:

> information readout• Role of transcription

• Structure of gene

• RNA polymerase – Transcription

• RNA processing

• Reference:

– Campbell; Chap 17



Role of transcription: copy of a gene



One gene – One protein

• Nucleotide sequence of a gene

– Provides the code for the amino acid sequence of a specificprotein

• Information from gene must be transferred toribosomes

– Where protein synthesis occurs

• Gene sequence is copied to mRNA

– Transcription



Transcription of genes

• Only the DNA of genes are transcribedinto mRNA

– Human genome: 3 x 109 bases

– 35,000 genes

– Only 5 % of DNA is composed of genes

• 5 % of DNA is transcribed into mRNA



mRNA (messenger RNA)

• Single stranded molecule

• Copy of one DNA strand from the gene• Sugar backbone is ribose

• Bases are A, G, C, or U – Compared to DNA: A, G, C, or T



The level of gene transcription is

regulated

• Multiple copies of mRNA can be made

• The amount of mRNA synthesizeddetermines the amount of protein produced

Gene 1 Gene 2

mRNA



General structure of a gene

• Promoter

– DNA sequence

– Binds RNApolymerase

– Determines start-siteof transcription

• Terminator

– DNA sequence

– Termination signal



Initiation of

transcription• Eukaryotic promoter

– TATA box

– Binds transcriptionfactor

– RNA polymerasethen binds



RNA polymerase

• What does it do ?

– unwinds the two strands of DNA

– Elongates (synthesizes) in 5’ to 3’ direction

– adds nucleotides to the 3’ OH end

– Copies from the DNA template strand

– mRNA produced is complimentary ratherthan identical to the template strand





Elongation

• RNA polymerase moves along the DNAand unwinds the double helix

– 10-20 bases at a time

• mRNA synthesis: 60 nucleotides / sec

• DNA double helix then reforms



Termination

• transcription continues until RNA polymerasereaches the termination signal

• in prokaryotes: stops immediately

• in eukaryotes: RNA polymerase continues

– 10-35 nucleotides past the termination signal

– addition of a poly A tail



Split genes and RNA splicing



RNA processing:

> eukaryotic cells• Modification of

mRNA transcript

– RNA splicing (cutand pasteprocedure)

– 5’ cap

– Poly(A) tail



Protein synthesis: Translation

• 3 types of RNA

– mRNA

– rRNA

– tRNA

• Triplet genetic code

• Translation

– overview

• Reference:

– Campbell; Chap. 17



3 types of RNA are involved in

Translation• mRNA (messenger RNA)

– Carries information from gene

• rRNA (ribosomal RNA) – Major component of ribosomes

– Location of protein synthesis (translation)

• tRNA (transfer RNA) – Carries amino acids to ribosome



Polyribosomes

• mRNA is translated by several ribosomes



Why a tripletgenetic code ?

4 nucleotides

20 amino acids

4 x 4 = 16

4 x 4 x 4 = 64



64 codons

61 code for Amino acids

3 codonsStop signal



The genetic code has

redundancy• An amino acid can be

specified by more than onecodon

• Nearly universal

• Non-overlapping



The reading frame:

> 3 possibilities; only one is correct

• Translation of mRNA – Must begin at the correct nucleotide

– All proteins start with Methionine

– Start codon is AUG



Mutations in DNA affect protein

structure• Mutation in DNA

code

• Change aminoacid sequence

– Change protein

activity



Structure of aribosome

• Two subunits

– Large / small

• Both composed ofproteins & rRNA

• Has 3 sites

– A: Aminoacyl-tRNA

– P: Peptidyl-tRNA

– E: Exit site



Structure & function of tRNA

• tRNA has a specific 3-D shape

• Each is linked to a specific amino acid – At one end of tRNA molecule

• Anticodon sequence – Located at other end

– Binds to the codon on mRNA strand





Translation:> the basic concept

• mRNA slides through theribosome

• Codons are bound byspecific tRNA-amino acid

• tRNA adds it’s amino acid

to growing polypeptidechain

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BIOL2146 DNA Replication and Gene Expression