3.9 DNA Replication & Genetic Code

Question 1

What is DNA replication?

Answer 1

Semi-conservative process

of producing identical copies of DNA molecules

Question 2

Outline the steps of DNA replication:

Answer 2

1. Double helix unwinds.
2. Helicase breaks hydrogen bonds between complementary DNA bases.
3. Free DNA nucleotides align with exposed bases by complementary base pairing.
4. Hydrogen bonds reform.

ON LEADING STRAND: (5’ to 3’ direction)

1. DNA polymerase joins adjacent nucleotides by phosphodiester bonds - continuous.

ON LAGGING STRAND: (3’ to 5’)

1. DNA polymerase joins adjacent nucleotides by phosphodiester bonds discontinously.
2. Forms Okazaki fragments, which are joined by ligase.
3. Sugar-phosphate backbone forms.
4. Each DNA molecule has 1 new & 1 old strand - semi-conservative.

Question 3

What enzymes are involved in DNA replication & what are their roles?

Answer 3

• HELICASE - breaks hydrogen bonds between complementary DNA bases.
• DNA POLYMERASE - forms phosphodiester bonds between adjacent nucleotides.
• LIGASE - joins Okazaki fragments on lagging strand

Question 4

The presence of a leading & lagging strand is due to DNA running in antiparallel, & the fact that nucleotides can only be added onto the ____ end.

Answer 4

3’ (to the hydroxyl)

Question 5

Nucleotides can only be added onto the ____ end.

Answer 5

3’ (to the hydroxyl)

Question 6

DNA replication occurs continuously in the ____ to ____ direction.

Answer 6

5’ to 3’

Question 7

Describe the characteristics of genetic code:

Answer 7

• UNIVERSAL
  
  • used by all organisms
• DEGENERATE
  
  • many codons for 1 amino acid
• PUNCTUATED
  
  • contains introns (non-coding) regions, e.g stop codon TAG, start codon ATG
• NON-OVERLAPPING
  
  • bases not shared between codons
• SPECIFIC
  
  • Each codon codes for only 1 specific amino acid.

Question 8

What are introns?

Answer 8

Regions of non-coding DNA

Question 9

What are exons?

Answer 9

Regions of coding DNA

Question 10

What is a mutation?

Answer 10

Change in sequence of DNA bases

may affect phenotype

Question 11

How do mutations arise?

Answer 11

Randomly & spontaneously

Question 12

What is a triplet code?

Answer 12

Sequence of 3 nucleic acid bases

which codes for an amino acid

Question 13

What is a gene?

Answer 13

Section of DNA

which contains complete sequence of bases

to code for a whole protein