Structure and Properties of DNA

Question 1

Formation of a single DNA strand (polynucleotide)

Answer 1

1)The 5’ - phosphate can react with the 3’-OH to form a phosphoester bond

2)The alternating sugar and phosphate are held together by phosphodiester bonds and forms a ‘backbone’

3)One end has a 5’ phosphate group while the other end has a 3’ hydroxyl group

Question 2

DNA often exists as a double strand molecule

Answer 2

Double-helix structure of DNA was first deduced by James Watson and Francis Crick in 1953

Question 3

1)Structure of DNA

Answer 3

1)Each DNA strand is often associated with a complementary DNA strand. The two DNA strands run antiparallel to each other and are twisted to form a double-helix structure

2)Both stands are held together by hydrogen bonds between the bases on complementary strands

3)The complementary bases on the DNA strands always pair as:
-A to T (2 hydrogen bonds)
-G to C (3 hydrogen bonds)

4)The backbone of each strand consists of alternating sugar and phosphate groups linked by phosphodiester bonds (covalent bonds are very stable)

5)The phosphate groups in the backbone are negatively charged and can form ionic interactions with positively charged proteins or hydrogen bonds with water

6)The intrastrand hydrogen bonds are non-covalent interactions and can break and reform easily. The breaking of these bonds is facilitated by high temperature, extreme pH, high concentrations of ions. The DNA is said to have ‘,melted’ or denatured’ when its hydrogen bonds are broken and the 2 strands are dissociated

Question 4

Why do purine bases from hydrogen bonds with it’s complementary pyrimidine bases?

Answer 4

A purine base forms hydrogen bonds with its complementary pyrimidine base (and vice versa) because the 2 molecules fit nicely to give a stable structure

Question 5

2)DNA Packaging and Chromosomes

Answer 5

1)Each human cell (except the gametes) has bout 6 billion nucleotides with a total length of about 1.8 to 2 meters

2)These are divided with 46 double-stranded DNA molecules and tightly packed into a nucleus of about 6 micrometers diameter by proteins known as histones

3)Each of these double-stranded DNA molecules is a chromosome

Question 6

What are chromosomes?

Answer 6

Thread-like structures consisting od a single molecule that is tightly coiled around proteins known as histones

Question 7

3)DNA carries/contains the genetic code for making proteins

Answer 7

1)The sequence of bases in DNA determines the amino acid sequence of a protein:
-DNA carries the code for making proteins
-Each region of DNA that codes for a protein is call a gene

2)The code for each protein on DNA must first be copied into an RNA known as Messenger RNA (mRNA)
-The process is known as Transcription and occurs in the nucleus

3)In eukaryotes, the mRNA leaves the nucleus to be ‘read’ by ribosomes into a polypeptide
-The process is known as Translation

4)DNA also code for functional RNA other than mRNA. These other functional RNA serves important functions in the cells. These are produced through transcription of genes but are not translated