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DNA
… and Replication
DNA
 DNA
is often called the
blueprint of life
 It contains the
instructions for making
proteins within the cell
 Full name:
deoxyribonucleic acid
Why do we study DNA?
 It’s
central
importance to life
on earth
 Medical benefits –
cures for diseases
 Better food crops
Shape
 DNA
is a very long
polymer
 It’s basic shape is
like a twisted
ladder or zipper
 This is called a
double helix
Nucleotides
 DNA
is a polymer
made up of smaller
monomers called
nucleotides
 Each nucleotide
contains a
phosphate group,
a deoxyribose sugar
and a nitrogen base
Nitrogenous bases
 DNA
has 4 nitrogen
bases:




Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
 The
bases are
specifically paired:


Adenine – thymine
Guanine –
cytosine
Base pairings
 The
nitrogen bases
are attracted to
each other through
hydrogen bonds
 Hydrogen bonds
are weak but there
are millions of them
in a single DNA
molecule
CHECKPOINT
Thumbs up = true, thumbs down = false
 Each
nucleotide consists of a phosphate, nitrogen
base and a protein
1
DNA molecule = 1 nucleotide
 There
are 2 strands in each DNA molecule which are
held together by hydrogen bonds between the
nitrogen bases
CHECKPOINT
Hold up 1, 2, 3 or 4 fingers:
 In
a DNA molecule, cytosine always
pairs with
 1- Itself (Cytosine)
 2- Guanine
 3- Thymine
 4- Adenine
The Code of Life…
 The
“code” of a gene on a chromosome
comes from the specific order of the
bases in the gene
 A T C G T A T G C G G…
DNA Replication

DNA must be replicated or
copied before a cells divides



This happens during S
(synthesis) phase of interphase
The DNA molecule produces 2
identical new complimentary
strands following the base
pairing rules (A-T & C-G)
Each strand of original DNA
serves as a template for the
new strand
Replication process
 An
enzyme called
helicase unwinds
and separates
(unzips) the 2 DNA
strands by breaking
the hydrogen
bonds between
the bases

Replication

Another enzyme called
DNA polymerase adds
complementary
nucleotides to each
original strand
This enzyme initially makes
about 1 in 10,000 base
pairing errors



Other enzymes come
through and proof read &
correct these errors
1 in 1 billion errors after
proofreading
http://www.pbs.org/wgb
h/aso/tryit/dna/shockwav
e.html
Replication
 Semi-conservative
theory:


The 2 strands of the
parental (original) DNA
molecule separate
and each functions as
a template for synthesis
of a new
complimentary strand
Half of the original
strand is saved
CHECKPOINT
 What
is the name of the enzyme that unzips
the DNA molecule?
 What
is the name of the enzyme that adds
new nucleotides to the template strand?
 Why
is DNA replication referred to as “semiconservative”?
RNA – Ribonucleic acid

Responsible for the movement
of genetic information from
the DNA in the nucleus to the
ribosomes (site of protein
synthesis) in the cytoplasm

Structure of RNA:


Made of repeating nucleotides
Single-stranded
Differences from DNA:
1. Sugar – ribose
2. Nitrogen base – there is
NO thymine, RNA has uracil
instead which then pairs with
adenine

**Bellringer - Copy this slide down on
to Pg. 29 of your notebook (under the
notes sheet from yesterday)**
Types of RNA

1. Messenger RNA (mRNA):
 Carries genetic
information from DNA in
the nucleus to the
cytosol

2. Transfer RNA (tRNA):
 Takes amino acids from
cytosol to the ribosomes

3. Ribosomal RNA (rRNA):
 Makes ribosomes where
proteins are made