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Molecular Genetics:
Chapter 12

Chromosomes contain many genes made
of DNA.

A gene is a segment of DNA that codes for a
particular trait.
GENES DO 3 CRITICAL THINGS –
1.
2.
3.
carry information from one generation to the
next
determine the inheritable characteristics of
an organism
can be easily copied
Deoxyribonucleic Acid (DNA)
DNA is a long thin molecule in a human
cell with over 6 billion nucleotides.
If a cell was the size of a basketball, the
DNA would be 40 miles long.
DNA is….
a long molecule made up of units called
nucleotides.
Determines the production of proteins.
A nucleotide has 3 parts:
1.
2.
3.
Deoxyribose Sugar
Phosphate Group
Nitrogenous Base
There are 4 kinds of
nitrogenous bases in DNA…..
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
PURINES – have a “double” ring structure
PYRIMIDINES – have a “single” ring structure
DNA looks like a twisted ladder….
the sides of the DNA ladder (called the BACKBONE)
are composed of alternating…
deoxyribose sugars
and
phosphate groups
(covalently bonded together)
the steps of the DNA (called the RUNGS )
are composed of ….
complementary pairs
of nitrogenous bases
(covalently bonded to the sugar)
“untwisted”
“twisted”
The nucleotides
can be joined in any order….
this means that, any sequence of nitrogenous
bases is possible…
ACGGATACGATTAC or GACTATGATTCATA or
any other possible combination
How does this help to explain
the diversity of species?
Watson and Crick – determined the structure
of DNA as a DOUBLE HELIX in which two
strands were wound around each other
A double helix
looks
like a
twisted ladder
or a
spiral staircase.
HYDROGEN BONDS
•
weak bonds
•
do not involve the electrons
•
form between the N bases in DNA
•
provide just enough force to hold the two
strands of DNA together
HYDROGEN BONDS
can only form between certain base pairs
ADENINE and THYMINE
GUANINE and CYTOSINE
this is called
complementary base pairing

Chargaff’s Rules

for….

every adenine
 there is
 1 thymine

every guanine
 there is
 1 cytosine
COVALENT BONDS
 Occur when 2 atoms share electronsstronger than H bonds.

Found between



Sugars and phosphates
Sugars and nitrogen bases
Maintain the


Backbone (sides) of the DNA molecule
The integrity of the DNA code (sequence of N
bases)
DNA Structure
10 base pairs make up one full twist of
DNA.
What is the complement?
A T C G G C T T A A T A T A T C G
T A G C C G A A T T A T A T A G C


During most of the cell cycle, chromosomes
Are not visible, instead the DNA is seen in a
form called CHROMATIN
During cell division, the chromatin condenses and coils
around proteins (called histones) to form
CHROMOSOMES
Semiconservative DNA Replication
making an exact copy of DNA
occurs before cell division
highly accurate

mutations may occur = change in the
sequence of nucleotides = a change in the DNA
Watson and Crick
realized that each strand of a DNA molecule
has all the information needed to
RECONSTRUCT THE OTHER STRAND
A Simplistic View….
During Semiconservative DNA
replication…….
1.
2.
DNA molecule separates at H bonds
complementary DNA nucleotides attached
to exposed N bases
PRODUCES ….
2 new complementary strands following the
rules of base pairing (Chargaff’s Rules)
using SEMICONSERVATIVE replication
Semiconservative DNA replication requires enzymes
(proteins, catalysts to speed up the process)
DNA helicase - “unzips” the DNA by breaking the H
bonds between the complementary base pairs
DNA polymerase - “proofreads” each new DNA strand,
helping to maximize the odds that each molecule is a
perfect copy of the original DNA
DNA ligase – connects together the Okazaki fragments
on the lagging strand
Each strand of
DNA serves
as a template
(pattern) for the
new strand.
Product=2 new
complementary
strands of DNA
following the
rules of base
pairing.
Prokaryotes….





Have no nucleus
Have no membrane bound organelles
DNA is in the cytoplasm
Have a single, circular chromosome
DNA replication begins at a single point
on the chromosome and proceeds,
often in 2 directions until the entire
chromosome is replicated
Eukaryotes….





Have a nucleus
Have membrane bound organelles
DNA located in nucleus
DNA replication occurs at hundreds of places
and produces segments called Okazaki
Fragments.
Replication proceeds in both directions until
the DNA is completely copied.
Each DNA
molecule that
results from
replication has
one original
strand and one
new strand.


1 molecule of DNA
=
2 strands of DNA
The site where
separation and
replication occur are
called replication
Forks.

DNA is usually in a form called Chromatin

During cell division, the chromatin
condenses and couls around the proteins
(called histones) to form chromosomes (of
DNA and protein).

STOP
Ribonucleic Acid (RNA)…
•
•
•
•
organic compound (contain C+H)
polymer - composed of many monomers called
nucleotides
composed of a SINGLE STRAND of nucleotides
carries out protein synthesis (the making of proteins)
3 types of RNA….



messenger RNA (mRNA) : single chain
transfer RNA (tRNA): tee-shaped
ribosomal RNA (rRNA): globular
Ribonucleic Acid (RNA)
Ribosomal RNA
Messenger RNA
Transfer RNA
RNA takes on 3 different shapes. All are used in protein synthesis.
RNA nucleotide has 3 parts…
1.
2.
3.
a ribose sugar
covalently
a phosphate group
bonded
a nitrogen-containing base – A, U (uracil), C,G
In RNA – uracil takes the place of thymine
In RNA…. A pairs with U
C pairs with G
RNA nucleotide structure:
Nitrogen Base (A, U, C, G)
Phosphate Group
Ribose Sugar
RNA differs from DNA:



1. RNA has ribose sugar
(instead of deoxyribose)
2. RNA is a single strand of nucleotides
(instead of a double helix)
3. RNA has the N base uracil
(instead of thymine)
Transcription…the process of
making RNA from DNA
template
2 steps:
1.
an enzyme separates N-bases of DNA
2.
a complementary RNA chain is made
3 products:

mRNA, tRNA, rRNA
RNA transcription
The process of making RNA from DNA
2 steps:


1.
2.
RNA polymerase separates N-bases of DNA
Complementary chains are made
3 products:


mRNA, tRNA, rRNA
Messenger RNA (mRNA)
•
carries the genetic
message from nucleus
into cytoplasm
•
3 nitrogen bases = codon
•
codons code for specific
amino acids
Transfer RNA (tRNA)
•
transfers amino acids to
the site of protein
synthesis (ribosome)
•
anti-codon = 3 N bases
•
anti-codons align with
codons on mRNA
Ribosomal RNA (rRNA)
•
makes up ribosomes
•
this is where protein
synthesis occurs
•
the exact process is
unknown
Importance of Protein…
Your body uses the proteins you eat
(the amino acids you ingest)
to make specialized proteins
that have specific jobs!!!

LIKE…. Insulin, Actin, Hemoglobin, Collagen
and Elastin, Pepsin and Trypsin and other
enzymes, Antibodies and many, many
more…..
You need protein !!!!
Translation….the making of proteins
from the information encoded in DNA
this process makes ALL types of proteins
amino acids  polypeptide chain  protein
occurs at a ribosome (site of protein synthesis)
In this process all 3 types of RNA are used . . . . .

mRNA carries the instructions for a protein

tRNA transfers amino acids to the ribosome

rRNA assists in the binding of mRNA and tRNA
**proteins made in cytoplasm will stay in the cell and those
made on the (rough) ER will be shipped out of the cell
Steps in Translation
1.
2.
3.
4.
5.
mRNA leaves nucleus and goes to ribosome (rRNA)
tRNA brings amino acids to ribosome (rRNA)
codons + anticodons align = bring amino acids into place
several amino acids = polypeptide chain
polypeptide chains will detach and wrap with other chains
to form a protein
* process only begins with a start (AUG) codon
* process will only stop with a stop codon (UGA, UAG,…)
A look at
translation…
Let’s look
at it again
To summarize the whole process
1.
Replication
DNA  DNA
2.
Transcription
DNA  RNA
3.
Translation
RNA  Protein
RNA Editing – RNA molecules require
a bit of editing before they are ready
to go into action

Introns – Deleted sequence of RNA

Exons – Expressed sequence of RNA
The Genetic Code
amino acids
polypeptide chains
proteins
the genetic code is read 3 letters at a time
example - AUG, CAA, UCG, ACC, GAC
each sequence of 3 letters “codes” for
a specific amino acid
as amino acids are put into a specific order
they produce a specific type of protein
MUTATIONS – changes in the
DNA (nucleotide) sequence
Mutations are of 2 kinds:
• GENE MUTATIONS
• CHROMOSOME MUTATIONS – we have
already discussed these
GENE MUTATION
•
a mutation in a single gene (segment of DNA)
it may involve one or several nucleotides

point mutation – mutations that affect 1 nucleotide

frameshift mutation – a point mutation caused by the
ADDITION or DELETION of a single nucleotide results in shift of the “reading frame” of the genetic
code
AUG UUA CCA UGA
•

What happens if URACIL is added in front of ADENINE?
 What happens if ADENINE is deleted ?
Gene Regulation



A gene is expressed or “turned on” only if
transcription occurs
Operon – a group of genes that operate
together
Operator – region of the chromosome in
an operon to which the repressor binds
when the operon is “turned off”
Special Genes

Hox Genes – genes that determine an animal’s basic body plan – mutation
in one of these genes can completely change the organs that develop in
specific parts of the body

Oncogenes – genes that promote uncontrolled cell division which may lead
to cancer – genetic mutations may result to the activation of oncogenes

Tumor Suppressor Genes – genes that act to prevent DNA damage and
inhibit uncontrolled cell division which may prohibit cancer development
ONCOGENES AND TUMOR SUPPRESSOR GENES ARE
THOUGHT TO BE IN A PERPETUAL TUG-OF WAR
REMEMBER – A GENE IS NOT “TURNED ON” UNTIL IT
GOES THROUGH TRANSCRIPTION