Structural Properties of The DNA

BIOLOGY NOTES
SCASA ELABORATION #1
the structural properties of the DNA molecule, including nucleotide
composition and pairing and the hydrogen bonds between strands
of DNA, allow for replication.
STRUCTURE/ PROPERTIES OF DNA
DNA stands for Deoxyribonucliecacid.
It is made up of molecules called nucleotides.
DNA is a molecule that contains instructions an organism needs to
develop, live and reproduce. Production of Proteins.
These instructions are found everywhere, and are passed from
parents to children.
DNA consists of two strands in the shape of double helix/ ladder.
Attached to it are nucleotides to form two long strands that spiral
creating a double helix.
NUCLEOTIDE COMPOSITION
Nucleotides are made up of sugar (deoxyribose), a phosphate group
and nitrogenous base.
Nitrogenous bases are Adenine, Thymine, Guanine and Cytosine.
Adenine pairs with Thymine and Guanine pairs with Cytosine.
The phosphate and sugar molecules are sides of double helix and
the rungs are made up of nitrogenous bases.
Amount of guanine is the same for cytosine and the same amount
for adenine and thymine.
Called the complementary pairs – guanine always hydrogen bond
with cytosine and adenine always hydrogen bond with thymine.
These pairing help produce 3D helical structure of DNA.
ANTIPARALLEL
The two strands run in opposite directions to each other and are
called antiparallel and twisted into a double helix.
Nucleotides on opposite strands pair.
SUGAR PHOSPHATE BACKBONE
Each phosphate group is attached to two sugar molecules by ester
bonds and is called a phosphodiester bond. The five carbon atoms in
sugar molecules which form a ring are numbered 1 to 5.
One ester bond is formed from 3 carbon and another from 5 carbon
of the next sugar ring. The chain of alternating sugar molecules and
phosphate groups is called sugar phosphate backbone.
DNA synthesis occurs in 5 to 3 directions.
HYDROGEN BONDS

The base pairs on the rungs of the ladder held together by hydrogen
bonds.
The hydrogen bonds break during DNA replication.
Adenine with Thymine shares two hydrogen bonds while Guanine
with Cytosine shares three hydrogen bonds.
FEATURES OF DNA
DNA is able to encode large amount of information (function)
DNA is chemically stable
It is able to accurately replicate itself
it controls and directs protein synthesis (function)
 Occasional mutations occur.
DNA REPLICATION
Is the process by which DNA makes a copy of itself during cell
division.
The purpose is to duplicate the code it carries. The code can then be
passed to daughter cells. For preparation of mitosis and meiosis
processes.
The antiparallel nature of DNA and the direction that DNA
polymerase functions (they add free nucleotides in a 5'-3' direction)
influences how the leading and lagging strands are replicated.
Occurs in the S phase of Interphase during Cell Cycle
SEMI - CONSERVATIVE
Consists of one parental strand and one new strand. It is one of two
strands conserved or retained from generation to next while the
other is a new strand.
DNA PROCESS
The first step is to unwind (unzip) the double helix structure of the
DNA molecule.
Each of two strands is copied, acting as a template, becoming half
of the new DNA molecule.
New strand is complementary to original strand. A - T AND C - G.
It starts with an enzyme called DNA helicase unzipping the long
molecule of double stranded DNA by breaking the weak bonds
between nucleotides, exposing nucleotide bases.
Hydrogen bonds holding two strands of DNA together are weak and
the enzyme is able to separate them easily.
This separation creates a ‘Y’ shape called a replication fork.
+Replication FORK is the junction between unwounded single strand
and intact double helix. It moves along parental DNA strands, so
that it can unwind the parental strands.
One of the strands is oriented in the 3’ to 5’ direction (towards the
replication fork), this is the leading strand. The other strand is
oriented in the 5’ to 3’ direction (away from the replication fork),

this is the lagging strand. As a result of their different orientations,
the two strands are replicated differently:
Once all of the bases are matched up (A with T, C with G), an
enzyme called exonuclease strips away the primer(s). The gaps
where the primer(s) where are then filled by yet more
complementary nucleotides.
The new strand is proofread to make sure there are no mistakes in
the new DNA sequence.
Finally, an enzyme called DNA ligase? seals up the sequence of DNA
into two continuous double strands.
The result of DNA replication is two DNA molecules consisting of one
new and one old chain of nucleotides. This is why DNA replication is
described as semi-conservative, half of the chain is part of the
original DNA molecule, half is brand new.
Following replication the new DNA automatically winds up into a
double helix.
A short piece of RNA ?called a primer? (produced by an enzyme
called primase) comes along and binds to the end of the leading
strand. The primer acts as the starting point for DNA synthesis.
DNA polymerase? binds to the leading strand and then ‘walks’
along it, adding new complementary? nucleotide?bases (A, C, G
and T) to the strand of DNA in the 5’ to 3’ direction.
This sort of replication is called continuous.
4. Lagging strand:
4. Numerous RNA primers are made by the primase enzyme and bind
at various points along the lagging strand.

4. Chunks of DNA, called Okazaki fragments, are then added to the
lagging strand also in the 5’ to 3’ direction.
4. This type of replication is called discontinuous as the Okazaki
fragments will need to be joined up later.
SCSA Elaboration
the genetic code is a base triplet code; genes include ‘coding’ and
‘non-coding’ DNA, and many genes contain information for protein
production
Proteins, including enzymes and structural proteins, are essential to
cell structure and functioning
Protein Synthesis
Genome: it is a complete set of genetic instructions for an organism.
The genome of an organism is composed of coding and non-
coding DNA.
Describe the function of each.
NON CODING DNA
→ they do not code for amino acids
→ most of them lies between genes on the chromosome
→ plays in the role of gene regulation.
→ does not provide instruction for the production of proteins
CODING DNA
G
Relationship between Genetic Code and Protein
A gene is a basic unit of heredity in a living organisms that resides
in the long stranded DNA called chromosomes
Functions of Proteins
It helps repair and build your body's tissues, allows metabolic
reactions to take place and coordinates bodily functions. In addition
to providing your body with a structural framework, proteins also
maintain proper pH and fluid balance.
→ growth and maintenance

→ causes biochemical reaction
Types of Mutation: Gene Mutation
Gene Mutation: is where one gene's mutates could possibly affect the proteins produced.
Point Mutation: is a type of gene mutation that changes the base sequence on one gene and
can form a new allele if not a silent mutation.
Single Nucleotide Polymorphism: a single nucleotide difference that occurs at a given
position in the genomes of two or more individuals.
3 types of substitution mutation
Silent: a mutation in which the DNA codon for one amino acids becomes another
DNA codon for the same amino acids.
Missense: a gene mutation that results in one amino acid being replaced by another
amino acid in the encoded protein.
Nonsense: a mutation in which a codon for an amino acid is changed to one that
codes for a stop codon, terminating translation.
Deletion and Insertion
BIOLOGY - MUTATION
SCSA ELABORATION
Mutations in genes and chromosomes can result from errors in DNA replication or
cell division, or from damage by physical or chemical factors in the environment
Mutation: a permanent change in the DNA sequence of a gene, an only source of new
alleles in a population’s gene pool; the process of generating a mutation.
Distinguish between Somatic and Germ - Line Cells
SOMATIC CELLSGERM LINE CELLS
Are any cells that are not involved in the
production of gametes.
Cells that create reproductive
cells or gametes.
Arranged into different types of tissues in
body of multicellular organisms,
Produce male and female
gametes to participate in sexual

performing specific functions reproduction.
Undergoes Mitosis Undergoes Meiosis
Mutation may not pass through the
generations and have no effect on
evolution
Mutation passes through the
generation having an effect on
evolution.
How can mutation be passed onto offspring?
A mutation can change the genetic sequence. Some mutations are hereditary
because they are passed down to offspring from a parent carrying a mutation through
the germ line - an egg or sperm carrying mutations. Mutation can affect sex cells
called gametes and can be inherited and incorporated into every cell of offspring.
BIOTECHNOLOGY
Describes the use of living things to make new products
E.g. Golden Rice, GM Cotton
Genetic Engineering: refers to the process in changing the genetic
sequence of an organism using modern biotechnological