6.4.4 Proofreading, End Replication, and Telomeres

Question 1

note

Answer 1

• DNA polymerase synthesizes new nucleotides with remarkable accuracy. For example, the human genome is extensive, and the nucleotide sequence—if printed out—would comprise 900 textbooks in an average font. Yet typically only one nucleotide in a billion is paired incorrectly.
  • DNA polymerase is responsible for genetic proofreading. It works in conjunction with ligase to excise and replace incorrectly matched base pairs.
  • The DNA in a eukaryotic cell is rod-shaped, which presents a problem in DNA replication. See the illustrations below to find out about the importance of telomeres in DNA replication.
• Review: DNA polymerase adds nucleotides only to the 3’ end of the existing strand.
• When a growing lagging strand reaches the end of the
  molecule, there is no way to add the last few DNA
  nucleotides. After the end RNA primer is removed, DNA
  polymerase cannot fill the gap in with DNA because DNA polymerase has no 3’ end to attach to. The result is that DNA molecules will shorten after each round of replication.
• Telomeres protect coding sequences of DNA. Eventually though, after many rounds of replication and shortening of the ends, the noncoding sequences will disappear, and important coding sequences will be degraded.
• Certain cell lines possess an enzyme called telomerase that can polymerize telomere sequences. Cells that replicate many times, such as germ cells, have telomerase.
• The experiments involving the cloning of the sheep, Dolly, shed light on the phenomenon of telomeres and cell aging. Cloning is the process of artificially producing a clone, a new individual genetically identical to an existing individual. To produce Dolly, a somatic cell from Dolly’s mother was combined with an unfertilized egg whose nucleus had been removed. At the time the experiment was performed, Dolly’s mother was six years old.
• Dolly is now two years old. The chromosomes of her cells, however, are eight years old because the original DNA from her mother was already six years old. Analysis of Dolly’s cells reveals older shortened chromosomes.
• Scientists now realize that the phenomenon of telomere shortening affects the viability of a cell. Researchers speculate that the addition of telomerase in somatic cells may increase the life span of a cell. The implications are longer life span and/or quality of life.

Question 2

Question: How can a cell afford to lose genetic material that codes for proteins critical to cell function?

Answer 2

Answer: The ends of DNA consist of long strands of
repeating nucleotides called telomeres that don’t code for cell products. The typical repeated unit in human DNA is TTAGGG.

Question 3

The accuracy of DNA replication is evident by the fact that on average only one nucleotide in a _______ is paired incorrectly.

Answer 3

• billion

Question 4

Which of the following statements concerning telomerase is true:

Answer 4

• A defect in telomerase would mean a gradual reduction of chromosome length with each replication cycle

Question 5

_________________ is when a somatic cell is combined with an unfertilized egg whose nucleus has been removed.

Answer 5

• Cloning

Question 6

Repair enzymes are most important for insuring the accuracy of which process?

Answer 6

• DNA replication.

Question 7

_______________________ is when an incorrect DNA sequence is cut out and replaced with the correct sequence.

Answer 7

• Excision repair

Question 8

Which one of the following statements concerning telomeres is correct?

Answer 8

• Telomeres protect the end of the lagging strand