3.9 DNA replication

Question 1

What is a simplified description of why DNA is replicated and what is an example of DNA replication?

Answer 1

• cells divide to produce more cells needed for growth or repair of tissues
• the two daughter cells produced as a result of cell division are genetically identical to the parent cell and to each other
• they contain DNA with a base sequence identical to the original parent cell

Question 2

What happens to a cell as it prepares to divide?

Answer 2

• the two strands of DNA double helix separates and each strand serves as a template for the creation of a new double-stranded DNA molecule

Question 3

What needs to be in place for DNA replicate?

Answer 3

• the double helix structure has to unwind and then separate into two stands so the hydrogen bonds holding the complementary bases together must be broken

Question 4

What happens in DNA replication after the hydrogen bonds have been broken apart?

Answer 4

• free floating activated DNA nucleotides will then pair with their complementary bases, which have been exposed as the strands separate
• hydrogen bonds then form between them

Question 5

What is the final step of DNA replication?

Answer 5

• the new nucleotides join to their adjacent nucleotides with phosphodiester bonds

Question 6

What does each new molecule of DNA include?

Answer 6

• one old strand of DNA
• one new strand of DNA

Question 7

What is the correct name of the process DNA replication?

Answer 7

• semi-conservative replication

Question 8

What is DNA replication controlled by?

Answer 8

• enzymes

Question 9

What are enzymes?

Answer 9

• a class of proteins that act as catalysts for biochemical reactions

Question 10

What is the limiting factor of enzymes?

Answer 10

• are only able to carry out their function by recognising and attaching to specific molecules or particular parts of the molecule

Question 11

What enzyme needs to be used before replication can begin?

Answer 11

• the unwinding and separating of the two strands of the DNA double helix
• DNA helicase

Question 12

What does DNA helicase do?

Answer 12

• it travels along the DNA backbone, catalysing reactions that break the hydrogen bonds between complementary base pairs as it reaches them
• this can be though of as the strand ‘unzipping’

Question 13

What is the second enzyme used towards the end of DNA replication?

Answer 13

• DNA polymerase
• catalyses the formation of phosphodiester bonds between these nucleotides

Question 14

What direction doe DNA polymerase move in and why?

Answer 14

• it can only bind to the carbon 3’ end so will always travel 3’ to 5’
• it only unwinds and unzips in one direction
• it has to replicate each of the template strands in opposite directions

Question 15

What is continuous replication?

Answer 15

• the strand that is unzipped from the 3’ strand can be continuously replicated as the strands unzip
• this strand is called the leading strand

Question 16

What is discontinuous replication?

Answer 16

• the other strand is unzipped from the 5’ end, so DNA polymerase has to wait until a section of the strand has unzipped and then work back along the strand
• DNA is then produced in sections (Okazaki fragments), which then have to be joined
• the strand is called the lagging strand

Question 17

Where could incorrect sequencing occur?

Answer 17

• in the newly-copied strand

Question 18

What is a mutation?

Answer 18

• errors that occur randomly and spontaneously and lead to a change in the sequence of bases

Question 19

Proteins are the foundation for …

Answer 19

• the different physical and biochemical characteristics of living things

Question 20

What are proteins made of and what is there structure?

Answer 20

• a sequence of amino acids
• folded into complex structures

Question 21

What does the genetic code, code for?

Answer 21

• DNA codes for a sequence of amino acids

Question 22

What does the instructions that DNA carries contain?

Answer 22

• the sequence of bases along the chain of nucleotides that make up the two strands of DNA

Question 23

What is the code in base sequences called?

Answer 23

• the triplet code

Question 24

What is the triplet code?

Answer 24

• a sequence of three bases called a codon
• each codon codes for an amino acid

Question 25

What is a gene?

Answer 25

• a section of DNA that contains the complete sequence of bases (codons) to code for an entire protein

Question 26

The genetic code is universal, what does this mean?

Answer 26

• that all organisms use this code
• although the sequences of bases coding for each individual protein will be different

Question 27

What is the codon that acts like the start codon when it comes at the beginning of the gene?

Answer 27

• signals the start of the sequence that codes for a protein

Question 28

What do the three ‘stop’ codons do?

Answer 28

• do not code for any amino acids and signal the end of the sequence

Question 29

What does the middle codon code for?

Answer 29

• the amino acid methionine

Question 30

What does having a single codon to signal the start of a sequence do?

Answer 30

• ensures that the triplet of bases (codons) are read ‘in frame’

Question 31

How many different amino acids that regularly occur in biological proteins?

Answer 31

• 20
• as there are a lot more codons than amino acids

Question 32

What does a degenerate code mean?

Answer 32

• many amino acids can be coded for by more than one codon