BIO122 CH 17

Central Dogma

The process of transferring genetic information from DNA to RNA to protein.

Gene Expression

The process by which DNA directs protein synthesis, including transcription and translation.

Transcription

The synthesis of RNA using the information in DNA, producing mRNA, tRNA, and rRNA.

Translation

synthesis of a polypeptide (later folded into a protein) using mRNA

Proteins

are the links between genotype and phenotype 

Genetic Code

The information content of genes, represented by the specific sequence of nucleotides that correspond to amino acids.

What does it mean to say the genetic code is redundant but not  
ambiguous? 

The genetic code has multiple codons for a single amino acid (redundant) but no codon specifies more than one amino acid (not ambiguous).

What does it mean to say the genetic code is universal 

The genetic code is the same for all living organisms on Earth.

What are the three steps of transcription?

Initiation, Elongation, and Termination

Initiation of Transcription

promoter (TATA Box) and transcription factors  

All types of RNA (mRNA, rRNA, and tRNA) are made by what?

transcription  

Initiation of Transcription: Transcription factors purpose? 

• Transcription factors mediate the binding of RNA polymerase and the initiation of transcription  

• Transcription factors work with TATA Box.   

Initiation of Transcription: the name of the promoter in eukaryotes 

TATA Box

Initiation of Transcription: promoter’s purpose? 

promoter tells DNA polymerase where to attach

What happens in Elongation of transcription? 

As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time.   

What happens in Termination of transcription? 

• Polyadenylation sequence  

• Termination occurs when RNA polymerase transcribes sequence of DNA known as a terminator  

• Differs in bacteria and eukaryotes   

Termination of transcription in bacteria? 

Bacteria: Termination at signal   

Termination of transcription in eukaryotes? 

Eukaryotes: RNA polymerase II transcribes a particular sequence, and The RNA transcript is released 10–35 nucleotides past this point   

What is meant by the template strand? How can you determine which strand of DNA is the template strand?  

• template strand is what I'm going to use to transcribe my RNA; what is used as the template  

• non template strand the other strand of DNA  

Direction of Transcription

5’ to 3’ direction.

How is transcription initiated? What is needed for this to occur? 

For transcription to occur, must have a promotor: the DNA sequence where RNA polymerase attaches which signals the start point 

Transcription Unit

The stretch of DNA being transcribed 

What is needed for transcription to be terminated to occur? 

RNA polymerase transcribes sequence of DNA known as a terminator  

mRNA Processing

Preparing mRNA to leave the nucleus so it can be translated into a polypeptide 

Whys mRNA processing needed? 

mRNA is threaded through pores in nucleus, so we have to protect it from breaking

mRNA processing: addition of a 5’ cap 

The 5’ end receives a modified nucleotide 5’ cap  

mRNA processing: splicing out introns: spliced meaning 

the introns are removed, and the exons are joined together, creating an mRNA molecule with a continuous coding sequence  

mRNA processing: splicing out introns occurs before… 

mRNA leaves the nucleus 

mRNA processing: splicing out introns carried out by what? 

RNA splicing carried out by spliceosomes   

mRNA processing: splicing out introns: spliceosomes consist of? 

a variety of proteins and ribozymes (small RNA molecules capable of acting like an enzyme)

mRNA processing: splicing out introns: noncoding regions meaning… 

Intervening sequences, or introns   

mRNA processing: splicing out introns coded regions meaning… 

Coded regions are exons because they are eventually expressed

What type of RNA is needed for translation?  

mRNA

What are the three steps of translation?

Initiation, Elongation, and Termination

What happens in Initiation of Translation? 

Ribosomes assemble around target mRNA and the start codon 5’ AUG is recognized.  

What happens in Elongation of Translation? 

• Codon recognition*, peptide bond, translocation*  

• The tRNA transfers an amino acid to the tRNA bound to the next codon, forming a peptide bond between the two amino acids.  

What happens in Termination of Translation? 

The ribosome recognizes the stop codon and the polypeptide is released.  

Transcription: where? 

nucleus

Translation: where? 

cytoplasm

Transcription: RNA? 

MRNA transcribed from DNA  

Translation: RNA? 

TRNA translates mRNA to amino acids using ribosome (made of rRNA)  

Are the steps for Transcription and Translation the same? 

Yes, but what happens in each step is different for transcription and translation 

How is Initiation different in Translation and Transcription? 

Translation: Start Codon  

Transcription: TATA Box 

How is Elongation different in Translation and Transcription? 

Transcription: RNA Polymerase  

Translation: Codon recognition*, peptide bond, translocation*  

How is Termination different in Translation and Transcription? 

Transcription: Polyadenylation sequence  

Translation: Stop Codon  

Transcription: Final Product? 

MRNA strand with 5’ cap and poly-A tail  

Translation: Final Product? 

Chain of amino acids (primary structure of protein) 

What is a codon? 

A codon is a very specific sequence of three RNA bases 

How is a codon used? 

• Use codon as a code using genetic code(chart) determine what amino acid   

• Sequence of amino acids is a polypeptide chain  

How is an anti codon used?

Anticodon forms complimentary base pairs with mRNA 

What role do ribosomes play in translation? 

Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons 

Ribosomal structure Translation: 

• Two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)   

• Each ribosome has three binding sites for tRNA: E, P, and A 

What is the significance/function of the E, P and A site in a ribosome?  

1. The P site holds growing polypeptide chain  

2. The A site carries the next amino acid to be added to the chain   

3. The E site is the exit site 

How is translation initiated? What is needed for this to occur?  

• A small ribosomal subunit binds with mRNA and an initiator tRNA (anticodon UAC)  

• The ribosomal subunit moves along the mRNA until it reaches the start codon (AUG) which signals the start of translation   

Translation Initiation factors (proteins) role 

bring in the large subunit that completes the translation initiation complex  

What are the three steps of elongation in translation? 

• Each addition of amino acids involves proteins called elongation factors   

1. Codon recognition 

2. Peptide bond formation   

3. Translocation 

Which of the three steps of elongation in translation require energy? 

Codon recognition and translocation 

In what direction does translation proceed?  

Translation works in the 5’ to 3’ direction of mRNA; meaning anticodons are read in the 3’ to 5’ direction 

How does translation terminate? What is needed? 

Elongation continues until a stop codon (UAA, UAG, UGA) in the mRNA reaches the A-site of the ribosome 

What is a release factor?

When a stop codon is recognized, the A-site accepts a protein called a release factor   

What does a release factor do? Translation: termination 

• This causes the addition of a water molecule instead of an amino acid   

• The reaction releases the polypeptide, and the translation assembly comes apart   

What is a mutation?

Mutation: changes in the genetic material of a cell   

why is a mutation significant?  

Mutations to one or a few nucleotides can affect protein structure and function