3 - Central Dogma

What are the 3 stages occur in each of the following: DNA replication, transcription, and translation?

1. initiation

2. elongation

3. termination

What occurs in initiation?

DNA helix is opened, the DNA replication proteins are positioned

What occurs in elongation?

synthesis of the new DNA strands

What occurs in termination?

DNA synthesis is stopped

What are the steps of DNA replication initiation?

• begins at origin of replication

• 2 replication forks are formed

• DNA synthesis proceeds bidirectionally

When does DNA synthesis begin in eukaryotes?

at all origins at the same time (50,000 DNA replication origins)

What enzymes are involved in DNA replication initiation?

• helicase

• topoisomerase I

What is the role of helicase?

unwinds the DNA

What is the role of topoisomerase I?

prevents topological distortion by breaking the DNA strand

What structure prevents the two original strands from re-forming a double stranded molecule during initiation?

single-stranded DNA binding proteins

What enzyme is used in replication elongation?

• primase

• polymerase

What is the role of the primase?

generates short RNA strand that binds to the single-stranded DNA to initiate DNA synthesis

What is the role of DNA polymerase?

• reads the nucleotide sequence of the parental strand in the 3’ → 5’ direction and synthesizes new DNA strand 5’ → 3"‘

• adds nucleotides by forming phosphodiester bonds at the 3’ end of the sugar molecule

What is the leading strand in replication elongation?

continuous replication

What is the lagging strand in replication elongation?

• discontinuous replication

• short Okazaki fragments are formed and later linked together with ligase

What is the purpose of looping of the lagging strand?

places it in a position for 5’ → 3’ polymerization, allowing DNA polymerase to synthesized both strands simulataneously

What enzymes are involved in DNA replication termination?

• RNA hybridase

• DNA polymerase

• DNA ligase

How is the RNA primer removed after DNA synthesis is completed?

removed by RNA hybridase, DNA polymerase fills the gap

What is the role of DNA ligase?

seals the last two nucleotides together

What happens if DNA polymerases make mistakes?

they remove the mis-paired deoxynucleotides when proofreading and add the correct base

What occurs in transcription initiation?

RNA polymerase is positioned on the DNA of the gene transcribed

What occurs in transcription elongation?

RNA polymerase transcribes the DNA sequence of the gene into an RNA molecule

What occurs in transcription termination?

RNA polymerase is released from the DNA

How does RNA polymerase know where to bind on the promoter region of RNA?

transcription factors bind to the promoter, RNA polymerase binds to the general transcription facotr to form the transcription initiation complex

What is the role of RNA polymerase in transcription elongation?

“walks” along template strand of DNA

Which strand of DNA will have the same nucleotide sequence as newly synthesized mRNA?

coding strand/non-template strand

Which direction does the transcription initiation complex move?

moves along the non-coding strand

What is the role of RNA polymerase in transcription termination?

• transcribes a poly (A) signal

• mRNA is cleaved and then ~200 adenine residues are added to form a poly (A) tail

What is an RNA transcript considered after it is first made?

pre-mRNA, must be processed into mRNA before leaving the nucleus

How is pre-mRNA processed to mRNA?

• a cap is added to the 5’ end

• a poly (A) tail is added to the 3’ end

What is RNA splicing and when does it occur?

• spliceosome splices out introns and combines exons

• processing of pre-mRNA

Define codon

3 consecutive bases in mRNA that specify a particular amino acid from N-terminus to C-terminus

What codons mark the start and end of a protein?

• 1 “start” codon → AUG

• 3 “stop” codons

What are the 3 binding sights on ribosomes for tRNA binding?

• P site: peptidyl or donor site

• A site: aminoacyl or acceptor site

• E site: exit site

What occurs during translation inititation?

• initiator tRNA carrying methionine attaches to small ribosomal subunit

• complex walks along mRNA in 3’ direction to find start codon

• larger ribosomal subunit joins to form initiation complex at sight of start codon

What occupies the P site?

methionine-carrying tRNA

What binds to the A site?

the next aminocyl tRNA with complementary anticodon sequence

What bond connects amino acids to each other?

peptide bond

How may protein synthesis be affected due to change in DNA sequence?

• a different protein forms

• protein may not form at all

• less protein

• increased protein

• unaffected