28. Structure and Replication of DNA. Nucleolus

Question 1

Structure of DNA

Answer 1

Structure of DNA:

• double helix
• four types of nucleotides (adenine, cytosine, guanine and thymine)
• Phosphodiester links hold the phosphate-deoxyribose backbone together
• DNA double helix is joined by Hydrogen bonds between the nucleotides.
• DNA strands are antiparallel

Question 2

DNA polymerase?

Answer 2

DNA polymerases: enzymes that carry out all forms of DNA replication.

Question 3

Process of DNA replication?

Answer 3

The replication process:
•DNA replication starts at the ‘Origin of Replication’

•Helicase enzyme unwinds the double helix to expose two single DNA strands and create a replication fork
- Single-strand binding proteins prevent the fork from snapping back together

•Primase enzyme uses the original DNA sequence to synthesize a short RNA primer

•DNA polymerase begins to synthesize a new DNA strand by extending RNA primer in a 5’-3’ direction. Each parental DNA strand is copied by one DNA polymerase.
oOn the lagging strand the direction of synthesis is opposite to the direction of the growing replication fork.
olagging strand is synthesized in short segments known as Okazaki fragments.
oDNA Polymerase III is responsible for laying Okazaki fragments.

• RNA Helicase recognises RNA primers bound to the DNA template- removes the primers by hydrolyzing the DNA
• DNA polymerase can fill the gap left by RNA Helicase
• The DNA replication process is complete when the ligase enzyme joins the short DNA pieces together into one continuous strand.

DNA replication

Stage one
The DNA is unwound and unzipped. The helix structure is unwound. Special molecules break the weak hydrogen bonds between bases, which are holding the two strands together. This process occurs at several locations on a DNA molecule.

Stage two
DNA polymerase adds DNA nucleotides in a 5’ to 3’ direction. Complementary DNA nucleotides are added to the now exposed bases on both strands. Adenine pairs with thymine, thymine with adenine, cytosine with guanine and guanine with cytosine. A primer is needed to start replication.
Leading strand is synthesised continuously. DNA polymerase adds nucleotides to the deoxyribose (3’) ended strand in a 5’ to 3’ direction.
Lagging strand is synthesised in fragments. Nucleotides cannot be added to the phosphate (5’) end because DNA polymerase can only add DNA nucleotides in a 5’ to 3’ direction. The lagging strand is therefore synthesised in fragments. The fragments are then sealed together by an enzyme called ligase.

Stage three
The two new strands twist to form a double helix. Each is identical to the original strand.

Question 4

Points about DNA?

Answer 4

DNA is the molecule that holds the instructions for growth and development in every living thing. Its structure is described as a double-stranded helix held together by complementary base pairs.

DNA as a double stranded helix held together by complementary base pairs.

The basic units of DNA are nucleotides. These nucleotides consist of a deoxyribose sugar, phosphate and base.

The nucleotides are identical except for the base, which can be an adenine, thymine, guanine or cytosine.

These basic units are linked together to form strands by strong covalent bonds between the deoxyribose sugar of one nucleotide and the phosphate of the next nucleotide.

These strong bonds form a sugar-phosphate backbone.

The ends of the DNA strand are called the 5’ end, pronounced 5 prime end, at the phosphate end, and the 3’ end at the deoxyribose end.

Three versions of the DNA structure showing two strands linked together with the hydrogen bonds