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The structure of DNA is ideally suited to its function.
Objectives
Understand the nature of chromatin - how DNA and histones interact to form nucleosomes.
Know the basic structures of the 10 nm chromatin fiber and the 30 nm chromatin fiber.
Realise that chromosomes are single pieces of DNA packaged into chromatin.
Understandings
Nucleosomes help to supercoil the DNA.
DNA structure suggested a mechanism for DNA replication.
DNA polymerases can only add nucleotides to the 3 end of a primer.
DNA replication is continuous on the leading strand, and discontinuous on the lagging strand.
DNA replication is carried out by a complex system of enzymes.
Some regions of DNA do not code for proteins but have other important functions.
Applications
Rosalind Franklin’s and Maurice Wilkins’ investigation of DNA structure by X-ray diffraction.
Use of nucleotides containing dideoxyribonucleic acid to stop DNA replication in preparation of samples
for base sequencing.
Tandem repeats are used in DNA profiling.
Skills
Analysis of results of the Hershey and Chase experiment providing evidence that DNA is the genetic
material.
Utilisation of molecular visualisation software to analyse the association between protein and DNA within
the nucleosome.
DNA STRUCTURE
DNA structure suggested a mechanism for DNA replication.
,How is a single chain of DNA made up?
each strand is composed of a backbone of alternating phosphate and deoxyribose molecules.
these two molecules are held together by a phosphodiester bond or linkage.
in a condensation reaction: a phosphodiester bond forms between a hydroxyl group of the 3ʹ carbon of
deoxyribose and the phosphate group attached to the 5ʹ carbon of deoxyribose.
there will always be a free 5ʹ carbon end with a phosphate group attached and a free 3ʹ carbon end with a
hydroxyl group attached.
How are the two strands of DNA held together?
the two backbones/chains run in opposite directions: antiparallel.
one strand has the 5ʹ carbon on the top and the 3ʹ carbon on the bottom.
the other strand has the 3' carbon on the top and the 5' carbon on the bottom.
the two strands are attached by their nitrogenous bases which form hydrogen bonds.
adenine and guanine are double-ring structures known as purines.
cytosine and thymine are single-ring structures known as pyrimidines.
complementary base pairing occurs because of the specific distance between the two chains.
two hydrogen bonds link adenine and thymine.
three hydrogen bonds link cytosine and guanine.
, Chromosomes
Thread-like structures of DNA packaging.
Consequently, DNA structure suggests two mechanisms for DNA replication:
replication occurs via complementary base pairing.
replication is bi-directional (proceeds in opposite directions on the two strands) due to the antiparallel nature
of the strands.
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