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676 Chapter 20 NEL

Section 20.2 Questions

1. State the central dogma of molecular genetics.
   2.Describe the role of the following molecules in gene
   expression: ribosomes, mRNA, tRNA.
   3.The genetic code is read in groups of three nucleotides
   called codons. Explain why reading the code in pairs of
   nucleotides is not sufficient.
   4.The following is the sequence of a fragment of DNA:
   GGATCAGGTCCATAC
   Transcribe this sequence into mRNA.
   5.Using the genetic code, decipher the following mRNA
   sequence:
   5
   - AUGGGACAUUAUUUUGCCCGUUGUGGU - 3
   
   6.The amino acid sequence for a certain peptide is
   Leu–Tyr–Arg–Trp–Ser. How many nucleotides are
   necessary in the DNA to code for this peptide?


2. Identify which step in transcription would be affected and
   predict what would happen in each situation:
   (a) The termination sequence of a gene is removed.
   (b) RNA polymerase fails to recognize the promoter.

8.Construct a table to compare the processes of replication

and transcription. Remember to consider both similarities

and differences.

9.Distinguish between the following terms:

(a) P site and A site

(b) codon and anticodon

(c) start and stop codon

(d) DNA and RNA

10.Identify which ofthe following selections correctly lists the

anticodons for the amino acids threonine, alanine, and

proline:

A. ACU GCU CCA

B. ACT GCT CCA

C. TGA CGA GGT

D. UGA CGA GGU

11. Errors are occasionally made during the process of
    transcription. Explain why these errors do not always result
    in an incorrect sequence of amino acids. Describe at least
    two examples to illustrate your answer.

Table 4 Summary of Translation

Initiation

• Ribosome subunits (large and small) bind to the mRNA transcript, sandwiching the
  mRNA between them.


• The ribosome moves along the mRNA, reading the codons.


• Translation begins when the ribosome reaches the start codon, AUG.
  Elongation


• Through the genetic code, each codon specifies a particular one of the 20 amino acids
  that make up polypeptides.


• Transfer RNA (tRNA) molecules have an anticodon that is complementary to the codon
  in the mRNA. The tRNA carries the amino acid specified by the codon.


• The ribosome contains two sites, the A (aminoacyl) site and the P (peptidyl) site.


• When the start codon is in the P site, the first tRNA delivers methionine. Since the start
  codon codes for methionine, all polypeptides initially start with this amino acid.


• The second codon of the mRNA is exposed at the same time in the A site. When the
  tRNA delivers the second amino acid, a peptide bond is formed between the two amino
  acids.


• The ribosome shifts over one codon. The tRNA that delivered the methionine is released
  from the P site.


• When the ribosome shifts, the tRNA containing the growing polypeptide moves to the
  P site. A third amino acid, specified by the third codon, is brought in to the A site by the
  next tRNA. A peptide bond is formed between the second and third amino acid.


• Amino acids continue to be added to the polypeptide until a stop codon is read in the
  A site.

Termination

• The stop codons are UAG, UGA, and UAA. At this point the ribosome stalls.


• A protein known as the release factor recognizes that the ribosome has stalled and
  causes the ribosome subunits to disassemble, releasing the mRNA and newly formed
  polypeptide.