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Human Gene)cs
DNA Structure
The genome is distributed
along chromosomes,
which are made of
compressed and entwined
DNA
...
• The molecule that encodes gene)c
informa)on in the nucleus of cells
...
DNA
• Occurrence: DNA is present in all cells
...
• In prokaryotes, the DNA is held within an irregularly shaped
body in the cytoplasm called the nucleoid
...
(Circular-‐ Bacteria, mitochondria and plas)ds,
Coiled-‐ viruses and bacteriophages)
DNA
• Number: The number of DNA molecules in
eukaryo)c cells corresponds to the number of
chromosomes per cell
...
Sugar
2
...
Nitrogenous Bases
DNA
• Sugar: The sugar present in the DNA is called
deoxyribose
...
DNA
• Phosphoric Acid: A molecule comprising the
sugar joined to a base is called a nucleoside
...
• Phosphate group is aRached by
phosphodiester bond
...
DNA
• Nitrogenous Bases: These are nitrogen
containing organic ring compounds
...
They are of two types namely ADENINE and
GUANINE
DNA
• Pyrimidines: Single ringed nitrogen
compounds
...
• Adenine forms adenosine, a nucleoside, when aRached to
ribose, and deoxyadenosine when aRached to deoxyribose
...
Watson-‐Crick Model: DNA is a
double helix
• 1951 – James Watson learns about x-‐ray
diffrac)on paRern projected by DNA
Knowledge of the chemical structure of
nucleo)des
• 1953 – James Watson and Francis crick
propose their double helix model of DNA
structure
DNA: structure
"
1
...
3
...
5
...
7
...
DNA is double stranded"
DNA strands are antiparallel"
G-C pairs have 3 hydrogen bonds"
A-T pairs have 2 hydrogen bonds"
One strand is the complement of the other"
There are about 10 base pairs per turn of "
the helix
...
5 bp/turn of the helix "
9
...
11
...
One turn of the helix is 34 Å
...
4 Å apart
...
"
"
DNA
• DNA is a macromolecule
• Each DNA is formed of two polynucleo)de
chains
• The two chains spirally coiled together to form
a double helix
• Each chain is formed of many units called
nucleo)des (building blocks of DNA)
DNA
• A nucleo)de is formed of three components,
namely a phosphoric acid, a deoxyribose sugar
and a nitrogenous base
...
DNA
• A nucleoside is formed of a base and a
deoxyribose sugar
...
• Deoxyribose sugar + a nitrogenous base –
Nucleoside
• Phosphoric acid +Nucleoside – Nucleo)de
DNA
•
•
•
•
•
Adenine and Guanine –Purines
Cytosine, Thymine and Uracil – Pyrimidines
Purines linked together with pyrimidines
A T, G C
The amount of adenine is equivalent to the
amount of thymine and the amount of
guanine is equivalent to the amount of
cytosine
DNA
The two chains of DNA are
complementary to each other
...
DNA
• In a double helix the direc)on of the nucleo)des in one
strand is opposite to their direc)on in the other strand
...
• The asymmetric ends of DNA strands are referred to as the 5ʹ′
(five prime) and 3ʹ′ (three prime) ends, with the 5' end being
that with a terminal phosphate group and the 3' end that with
a terminal hydroxyl group
...
• The sugar and phosphate form the back bones
and base pairs form the horizontal rungs
...
DNA
• DNA can be damaged by many different sorts
of mutagens, which change the DNA
sequence
...
• The type of DNA damage produced depends
on the type of mutagen
Forces affecting the stability of the DNA double helix
• hydrophobic interactions - stabilize
- hydrophobic inside and hydrophilic outside
• stacking interactions - stabilize
- relatively weak but additive van der Waals forces
• hydrogen bonding - stabilize
- relatively weak but additive and facilitates stacking
• electrostatic interactions - destabilize
DNA
• A DNA sequence is called "sense" if its sequence is
the same as that of a messenger RNA copy that is
translated into protein
• The sequence on the opposite strand is called the
"an)sense" sequence
...
e
...
• With DNA in its "relaxed" state, a strand
usually circles the axis of the double helix
once every 10
...
• The set of chromosomes in a cell makes up its genome; the
human genome has approximately 3 billion base pairs of DNA
arranged into 46 chromosomes
...
• Transmission of gene)c informa)on in genes is achieved via
complementary base pairing
...
• This paRern is found in both strands of the DNA
...
Chargaff’s Rule
• Amount of adenine always equals thymine
• Amount of cytosine always equals guanine
• Amount of A+T together is independent of C
+G
• The two chains are held together by hydrogen
bonds between nitrogen bases
...
• The second rule holds that globally both %A ~ %T and %G ~
%C are valid for each of the two DNA strands
...
• The second of Chargaff's rules is that the composi)on of DNA
varies from one species to another; in par)cular in the
rela)ve amounts of A, G, T, and C bases