DNA Synthesis

What is DNA made of?

long chain of nucleotides, each containing a 5-carbon sugar, a phosphate group and a nitrogenous base.

What are the parts of a nucleotide?

1. 5-carbon (deoxyribose) Sugar 

2. Phosphate Group 

3. Nitrogenous Base (A,T,G,C)

Which bases are purines (2 rings)? 

Adenine (A) and Guanine (G) 

Which bases are pyrimidines (1 ring)? 

Cytosine (C) and Thymine (T)

Why does DNA have a uniform width?

Because a purine pairs with a pyrimidine (A with T, G with C).

What type of bond connects nucleotides in the backbone? 

Phosphodiester bonds between sugar and phosphate 

What gives DNA its directionality?

5’ end has a phosphate
3’ end has an OH group

New nucleotides are added to the 3’ OH.

What are Chargraff’s Rules?

1. The % of bases varies by species

2. %A = %T and %G = %C

What base pairing has a stronger bonding? Why?

G-C pairs: 3 hydrogen bonds (stronger)
A-T pairs: 2 hydrogen bonds

What is semi-conservative replication? 

Each new DNA molecule has: 
1 old strand and 1 new strand. 

What enzyme unwinds DNA?

Helicase

What enzyme prevents strands from re-pairing?

SSBs (single-stranded binding proteins)

What enzyme relieves tension from unwinding? 

Topoisomerase

What enzyme adds RNA primers?

Primase

What enzyme builds new DNA direction?

DNA polymerase III, synthesizes 5’ to 3’

Why can’t DNA polymerase start on its own?

It needs a 3’ OH provided by RNA primers

What enzyme removes primers? 

DNA polymerase I

What enzyme seals the backbone?

Ligase

What is the leading strrand? 

the DNA strand synthesized continuously, towards the fork

What is the lagging Strand?

DNA strand synthesized discontinuously in pieces called Okazaki fragments.

Why does the lagging strand form Okazaki fragments?

Because DNA polymerase works only 5’ to 3’, opposite the fork direction.

In each Okazaki fragment, what is the sequence of Enzymes? 

Primase to DNA polymerase III to DNA polymerase I to Ligase 

How do eukaryotic and prokaryotic replication differ?

Prokaryotes: 1 Origin, circular fast
Eukaryotes: many origins, liner, slow