Chapter 16.3, 16.4, 17.4, 17.5 – Gene Expression - Translation and mutations

Note that this reading guide covers the latter parts of both chapters 16 and 17.

OBJECTIVE: Understand that the genetic code is used by the cell to create polypeptides with amino acids in a particular order.

OBJECTIVE: Understand how triplet codons are used in specifying amino acids

OBJECTIVE: Recognize the different types of DNA mutations and how they affect the final protein.

Underline start and stop codons in the following sequence of mRNA:

Base number: 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 5’ T A C T C G C A U G T C A G G U A G T U G A T T C 3’

Transcribe and translate the following strand of DNA. This first translation will serve as a basis against which to compare other mutations.
- Use the template strand to figure out what the mRNA should be. Make sure you transcribe it in the correct direction
- Search for the start codon. This will set the reading frame (see definition of frameshift mutation on p.359)
- Write down the amino acid coded for by the start codon.
- Move in the 3’ direction along the mRNA to the next codon.
- Write down the next amino acid.
- Continue until you find a stop codon.
```
 Base number: 0                 1                   2
              1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7
 Coding:   5’ A T A T G A C C G G A T C C T C A T T C T A G A T T G 3’
 Template: 3’ T A T A C T G G C C T A G G A G T A A G A T C T A A C 5’
 mRNA:

 Protein:

 
```
The DNA strand below is almost the same as the strand in the previous question, except that there has been a mutation at base 17. Circle the mutation. Transcribe and translate, as you did above.
```
 Base number: 0                 1                   2
              1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7
 Coding:   5’ A T A T G A C C G G A T C C T C C T T C T A G A T T G 3’
 Template: 3’ T A T A C T G G C C T A G G A G G A A G A T C T A A C 5’
 mRNA:

 Protein:

 
```
- Compare this resulting protein with the protein in question 15.
- What type of mutation is this?
- Is it likely to be deleterious (harmful to the individual)?

Below is the same strand of DNA again except for a point mutation at base 9.

Base number: 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 Coding: 5’ A T A T G A C C T G A T C C T C A T T C T A G A T T G 3’ Template: 3’ T A T A C T G G A C T A G G A G T A A G A T C T A A C 5’ mRNA: Protein:

Compare the protein back to the protein in the first questions.
How is it different?
What type of mutation is this?
Do you think this is likely to be deleterious? Explain.

Here’s the same DNA again except that base 15 has changed relative to the original.

Base number: 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 Coding: 5’ A T A T G A C C G G A T C C G C A T T C T A G A T T G 3’ Template: 3’ T A T A C T G G C C T A G G C G T A A G A T C T A A C 5’ mRNA: Protein:

What type of mutation is this?
Is it likely to be deleterious? Explain.

In the following strand, a base has been inserted between bases 9 and 10.

Base number: 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 Coding: 5’ A T A T G A C C G T G A T C C T C A T T C T A G A T T G 3’ Template: 3’ T A T A C T G G C A C T A G G A G T A A G A T C T A A C 5’ mRNA: Protein:

What happened to the stop codon?
What type of mutation is this?
Do you think it is likely to be deleterious? Explain.

In the following strand, base 16 has been deleted.

Base number: 0 1 2 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 Coding: 5’ A T A T G A C C G G A T C C T A T T C T A G A T T G 3’ Template: 3’ T A T A C T G G C C T A G G A T A A G A T C T A A C 5’ mRNA: Protein:

What type of mutation is this?
Is it likely to be deleterious? Explain.

What do you understand by a ‘deleterious’ mutation?
If we compare ourselves to each other, do you think that most differences (mutations) are beneficial, neutral, or deleterious?
Which do you think will be more deleterious, a deletion or a translocation? Consider what each type of mutation does to the presence of the gene?

OBJECTIVE: Be able to describe the process of translation in prokaryotic and eukaryotic cells
In prokaryotes, the enzyme __________________ travels down the length of DNA, transcribing the DNA into _____________. As the transcript is made, __________________ attach to the mRNA transcript and start the manufacture of __________________. The sequence of amino acids is specified by the 3-base-pair _________________. Transfer RNA molecules bring ______________ to the ribosome where they are attached to each other to form a polypeptide.
Where in the cell does translation occur in prokaryotes? In Eukaryotes?
What is the final outcome of translation? What does it accomplish?
Draw a diagram that shows the tRNA, its anticodon, the codon on the mRNA, and the amino acid that it carries.
Your textbook describes a tRNA molecule carrying an amino acid as being ‘charged’. It is charged both in the sense that it is carrying something, but also energetically. What enzyme is required to do this ‘charging’? Is this an exergonic or endergonic reaction?
How does a tRNA molecule ‘know’ which amino acid to carry?
How many tRNAs are there? How is it that so few tRNAs can accommodate all 61 codons on the mRNA?
In which direction is the mRNA read?
Describe how translation is initiated. Into which site (E, P, or A) on the ribosome does the tRNA bind?
Describe the sequence of events that leads to the elongation of the polypeptide chain.
What is the signal that indicates that a ribosome should stop translating?
In Eukaryotes, what happens after a protein has been completely translated?
List the two places where polypeptides are synthesized.
How does a ribosome ‘know’ that it should dock with the rough ER?