Ch9

Question 1

Give one characteristic of DNA base. Describe them in detail respectively.

Answer 1

• two types of nitrogenous bases
  => purines
  - adenine (A) and guanine (G)
  - two joined carbon-nitrogen rings
  => pyrimidines
  - thymine (T) and cytosine (C)
  - only one carbon-nitrogen ring
  - T is replaced by uracil (U) in mRNA

Question 2

What are nucleotides made up of? What are DNA and RNA? What attaches to nucleotides?

Answer 2

• (nitrogenous) base + (pentose) sugar + phosphate
• DNA=deoxyribonucleotide
• RNA = ribonucleotide
• phosphate group is attached to C-5 of the sugar (5’ phosphate group)

Question 3

How are nucleotides joined together and what do they form? What does the first and last nucleotide have? What does each DNA chain have?

Answer 3

• nucleotides are covalently joined by 3’-5’ phosphodiester bond to form a sugar-phosphate backbone
• first: 5’ phosphate (-PO4 3-); last: free 3’ hydroxyl group (-OH)
• each DNA chain has polarity, 5’ end and 3’ end

Question 4

How to describe the structure of DNA? Briefly describe them.

Answer 4

• described in 3 levels
  => primary structure –> sequences of nucleotides are joined by phosphodiester bonds
  => secondary structure –> double helix of DNA maintained by hydrogen bonds between bases, hydrophobic interactions and Van der Waals force
  => tertiary structure –> DNA is further condensed and folded up with histone proteins into chromosomes

Question 5

How are secondary structure of DNA orientated (DNA double helix)? When does it make one turn? What pairs with what and what will be formed? What does DNA double helix contain?

Answer 5

• two DNA strands run in an antiparallel direction
• make one turn every 3.4nm (10bp per turn)
• A pairs with T by forming 2 hydrogen bonds and C pairs with G by forming 3 H-bonds
• 2 grooves: major (provide spaces for binding of DNA) and minor

Question 6

What does tertiary structure of DNA refer to and what is the function? How is it done?

Answer 6

• refers to the further coiling (supercoiling) of double helix into a highly compact structure called chromosome –> allow more DNA to be packed inside the nucleus
• done by winding DNA double helix around histone proteins –> nucleosome –> chromatin fibres –> chromosomes

Question 7

What are nucleosomes? What does it consist of? How does it do with histone proteins? What does another histone protein do?

Answer 7

• basic unit of chromatin fibres that condense to form chromosomes
• consists of DNA and histones
• a nucleosome consists of approximately 2 turns of DNA (146bp) wound around a core of 8 histone proteins: (H2A)2(H2B)2(H3)2(H4)2
• another histone protein (H1) binds DNA to histone octomer to stabilise the octomer

Question 8

What are chromatin made up of and what will chromatin form? What will happen during cell division? What do human somatic cells have? How are the chromosomes different?

Answer 8

• made up of nucleosomes (DNA+histone proteins)
• chromatin fibers
• chromatin fibers condense and chromosome becomes visible as distinct structure
• have a diploid number (2n) of chromosomes (paternal and maternal)
• 22 pairs of autosomes + 1 pair of sex chromosome (XX: female; XY: male) –> 46 chromosomes

Question 9

What are centromeres and telomeres essential? What are the specific functions of centromeres and telomeres? What is telomeres? Why will telomeres becomes shorter and what will happen if it’s too short?

Answer 9

• essential for the stability of eukaryotic chromosomes
• C: required for the segregation of the chromosomes during cell division
• T: highly repetitive DNA sequence (TTAGGG in human) at the chromosome ends –> ensure cell survival
• telomeres shorten with repeated cell division. If it’s too short –> cell death