1 2 3 4 Review: Proteins and their function in the early stages of replication 1 = initiator...
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Transcript of 1 2 3 4 Review: Proteins and their function in the early stages of replication 1 = initiator...
123 4
1 = initiator proteins2 = single strand binding proteins3 = helicase4 = topoisomerase (gyrase)
Replication
• Two DNA polymerase enzymes are necessary for replication in E. coli– DNA polymerase I – DNA polymerase III
- Along each template DNA strand, leading and lagging strands can be observed.
- The names were suggested based on synthesis at any given region.
- At any particular point in the DNA strand, if there is a leadingstrand, the complementary strand will have lagging strand.
Replication
• Two DNA polymerase enzymes are necessary for replication in E. coli– DNA polymerase I – DNA polymerase III
• Both have polymerase and exonuclease activities (functions)
• First let us take a look at the polymerase activity aspect of DNA polymerases and then discuss exonuclease activities
Replication• DNA Polymerase III
– Synthesize new DNA in the 5’ 3’ direction• Synthesizes long sequences of new DNA• Is highly processive; synthesizes DNA for a long period of time
without releasing the template• For example, synthesizes leading strand
• DNA Polymerase I– Synthesize new DNA in the 5’ 3’ direction
• Only synthesizes short sequences of new DNA• But before it could do this, it needs to remove RNA primers• This is achieved by its 5’ 3’ exonuclease activity
5’ 3’ exonuclease activity of DNA polymerase I
Replication
• The phosphodiester backbone of adjacent DNA fragments must be joined after DNA synthesis by DNA polymerases I and III
• This is done by the enzyme DNA ligase
Both DNA polymerases have proof readingactivity
This is a 3’ 5’ exonuclease activity
DNA Polymeraseactivity
Replication
• DNA Polymerase I– Synthesize new DNA in the 5’ 3’ direction
• Only synthesizes short sequences of new DNA
– 3’ 5’ exonuclease activity (proofreading)– 5’ 3’ exonuclease activity (remove primers)
• DNA Polymerase III– Synthesize new DNA in the 5’ 3’ direction
• Synthesizes long sequences of new DNA
– 3’ 5’ exonuclease activity (proofreading)
NOTE: DNA polymerase III does not have the 5’ 3’ exonuclease activity
This week we will complete…
Chapter 13 (transcription)Pages 348 – 361
Chapter 15 (translation)Pages 409 - 421
The Central Dogma(Francis Crick, 1958)
(Transcription) (Translation)
DNA RNA Protein(Gene/Genotype) (Phenotype)
An informational process between the genetic material (genotype) and the protein (phenotype)
Properties of RNA
RNA has the sugar ribose rather than deoxyribose
Properties of RNA
Nucleotides carry the bases adenine, guanine and cytosine (like DNA)
But uracil is found in place of thymine
Structure of RNA
• Designate the Nucleotides – Purines
• Guanine = G• Adenine = A
– Pyrimidines• Uracil = U• Cytosine = C
A phosphodiester bond
A phosphodiester bond
Structure of RNA
Nucleotides join together, forming a polynucleotide chain, by phosphodiester bonds
Usually single-stranded
Can have a much greater variety of complex three dimensional shapes than double-stranded DNA
Classes of RNA for Transcription and
Translation• Informational RNA (intermediate in the process of
decoding genes into polypeptides)– Messenger RNA (mRNA)
• Functional RNAs (never translated into proteins, serve other roles)– Transfer RNAs (tRNA)
• Transport amino acids to mRNA and new protein– Ribosomal RNAs (rRNA)
• Combine with an array of proteins to form ribosomes; platform for protein synthesis
– Small nuclear RNAs (snRNA)• Take part in the splicing of primary transcripts in
eukaryotes– Small cytoplasmic RNAs (scRNA)
• Direct protein traffic in eukaryotic cells– Micro RNAs (miRNA)
• Inhibits translation and induces degradation of complementary mRNA
RNA nucleotide sequences are complementary to DNA molecules
DNA template
New RNA is synthesized 5’ to 3’ and antiparallel to the template
DNA template Complementary RNA
Adenine UracilGuanine CytosineCytosine GuanineThymine Adenine
Synthesized 5’ to 3’ and antiparallel to the template
Only one strand of the DNA acts as a template for transcription
The template strand can be different for different genes
But….
For each gene only one strand of DNA serve as a template
Transcription
Catalyzed by the enzymeRNA polymerase
Single RNA polymerase (Prokaryotes)
Core enzyme Holoenzyme2 ,1 and 1 ’ subunits 2 , 1 , 1 ’ subunits plus
σ subunit
Polymerizes RNA Finds initiation sites
Initiation: The region that signals the initiation of transcription is a promoter
- 35 bases from initiation of transcriptionRecognized by RNA polymerase
- 10 bases from initiation of transcriptionUnwinding of DNA double helix begins here
Elongation: RNA is polymerized in 5’ 3’ direction
Elongation
NTPS (ATP, GTP, CTP, UTP) are added
The energy is derived by splitting the high-energy triphosphate bond
Termination
RNA polymerase recognizes signals (sequence) for chain termination
Releases the RNA and enzyme from the template
http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_1.html
Animation on Transcription