What is the function of DNA gyrase? O a prevents the two complementary template strands from bonding back to each other O b.reduces the torsional strain (knotting) that builds up ahead of the replication fork as a result of unwinding Oc connects Okazaki fragments Od umvinds the double helix at the replication fork

During proofreading, which of the following enzymes reads the DNA? primase topoisomerase DNA pol helicase

Multiple Replication Forks in E. coli II On the basis of Figure 28.2, draw a simple diagram illustrating replication of the circular E. coli chromosome (a) at an early stage, (b) when one-third completed, (c) when two-thirds completed, and (d) when almost finished, assuming the initiation of replication at oriC has occurred only once. Then, draw a diagram showing the E. coli chromosome in problem 3 where the E. coli cell is dividing every 20 minutes.

Review Figures 8.12 and 8.13. In cells, the primers for DNA synthesis are short strands of RNA, so each newly-synthesized strand of DNA has a segment of RNA al its 5 end. As replication proceeds, DNA polymerases remove these RNA segments and fill in the resulting gaps with DNA. However, the gaps at the very 5 ends of the new strands cannot be filled in with DNA. Why not? DNA replication leaves exposed about 100 nucleotides al the 5 end of each template strand, and these single-stranded ends are removed. What are the effects of this "end problem" on a cell's DNA as it continues to divide? FIGURE 8.12 DNA replication. Green arrows show the direction of synthesis for each strand. The Y-shaped structure where the DNA molecule is being unwound is called a replication fork. FIGURE 8.13 Discontinuous synthesis of DNA. This close-up of a replication fork shows that only one of the two new DNA strands is assembled continuously. The other is assembled in short segments.

Figure 14.14 You isolate a cell strain in which the joining of Okazaki fragments is impaired and suspect that a mutation has occurred in an enzyme found at the replication fork. Which enzyme is most likely to be mutated?

Transcribe and translate the following DNA sequence (nontemplate strand): 5'- ATGGCCGGTTATTAAGCA-3'

Human Genome Replication Rate Assume DNA replication proceeds at a rate of 100 base pairs per second in human cells and origins of replication occur every 300 kbp. Assume also that human DNA polymerases are highly processive and only two molecules of DNA polymerase arc needed per replication fork. How long would it take to replicate the entire diploid human genome? How many molecules of DNA polymerase does each cell need to carry out this task?

Using Figures 8.7 and 8.9 as a guide, draw a dinucleotide composed of C and A. Next to this, draw the complementary dinucleotide in an antiparallel fashion. Connect the dinucleotides with the appropriate hydrogen bonds. FIGURE 8.9 The two polynucleotide chains in DNA run in opposite directions. The left strand runs 5 to 3, and the right strand runs 3 to 5. The base sequences in each strand are complementary. An A in one strand pairs with a T in the other strand, and a C in one strand is paired with a G in the opposite strand. FIGURE 8.7 Nucleotides can be joined together to form chains caled polynucleotides. Polynucleotides are polar molecules with a 5 end (at the phosphate group) and a 3 end (at the sugar group). An RNA polynucleotide is shown at the left, and a DNA polynucleotide is shown at the right.

How does the double helix structure of DNA support its role in encoding the genome? The sugar-phosphate backbone provides a template for DNA replication tRNA pairing with the template strand creates proteins encoded by the genome Complementary base pairing creates a very stable structure Complementary base pairing allows for easy editing of both strands of DNA.

Multiple Replication Forks in E. coli I Assuming DNA replication proceeds at a rate of 750 base pairs per second, calculate how long it will take to replicate the entire E. coli genome. Under optimal conditions, E. coli cells divide every 20 minutes. What is the minimal number of replication forks per E. coli chromosome in order to sustain such a rate of cell division?