Molecular Genetics
There’s Two Critical Functions a Cell
must Perform during its Lifetime.
• 1. A Cell must divide (reproduce) and have
some way to ensure that each of the new
daughter cell is complete and somehow
receives the information it will need to
perform its life functions.
• 2. Once a cell has inherited the
information it needs from the mother cell, it
has to use that information to perform a
job…. And that job is to make Proteins.
Traits
• So…… why do some humans have blue
eyes, some green eyes, some brown eyes?
• Why do spiders have 8 legs?
• Why does a tree have white bark?
• We know how traits are passed from one
generation to the next, thanks to Mendel.
• We know some traits are dominate, some
are recessive, some are sex-linked, etc…
The 3 factors which determine
the traits of a organism are:
• 1. The absence or presence of a particular
protein... determined by the absence or
presence of an enzyme (these enzymes build
the structural proteins). These enzymes are
designed by the DNA.
• 2. The genes (DNA) the organism inherits
from its parents.
• 3. The environment... amount and type of
living conditions, food, light, etc...an
organism has its designed genetic potential
but might fail to obtain this potential because
environmental conditions are inadequate.
• But, what actually tells your
cells how to build you?, or a
tree cell to make a root or a
leaf?, or a jellyfish cell to make
a tentacle?…. What is this set
of instructions?
The answer of course is
DNA
DNA instructs the cell’s ribosomes to build the proteins
which are used to make all living organisms.
Actually, there’s two types of Nucleic Acids DNA and
RNA…. More about RNA later.
The Location of DNA
• In Prokaryotes
organisms without a
nucleus, or any
membrane-bound
organelle, DNA is
found near the center
of the cell in an area
called the “Nucleoid”.
• In Eukaryotes the
DNA is found in the
“Nucleus”.
Do You Remember?
• Carbohydrates are made of sugars.
• Lipids (Fats) are made of fatty acids.
• Proteins and Enzymes, which are Proteins, are
made of Amino Acids.
• Well…DNA and RNA are made of Nucleotides.
The Structure of DNA
DeoxyriboNucleic Acid
• DNA is a polymer composed of DNA Nucleotides.
Covalent Bonds
Each “Nucleotide” is made
up of 3 components:
1. A phosphate group
2. A sugar – the sugar in
DNA is Deoxyribose.
3. A Nitrogenous Base
Held Together by
“Covalent Bonds.” These
bonds are strong…. You
wouldn’t want your DNA
falling apart
This is how I draw a Nucleotide
P
S
B
A Nucleotide
The Phosphate Group is the
same as the phosphates
used to make ATP…. You do
remember ATP don’t you?
This Nitrogenous Base is
Adenine, it is one of 4
Nitrogenous Bases…..
Nucleotides are Identified by
their Nitrogenous Base.
The Sugar in DNA is
Deoxyribose and is a 5
carbon ring sugar.
The 4 Nucleotides are:
Adenine and Guanine are Purines. They have two Carbon Rings.
Adenine
Guanine
Cytosine and Thymine are Pyrimidines. They have only one Carbon Ring.
Cytosine
Thymine
The overall structure of DNA is
referred to as a “Double
Helix”
(Double Strand)
One Helix
the other Helix
the symbol 5’ - means 5 prime
the symbol 3’ - means 3 prime
* the symbols 5’ and 3’ will
become important to you later
The Nitrogenous Bases are Paired
Adenine always bonds with Thymine
or
A T
The Nitrogenous Bases, Adenine and Thymine, are held together by 2 Weak
Hydrogen Bonds
The Nitrogenous Bases are Paired
Guanine always bonds with Cytosine
or
G C
The Nitrogenous Bases, Guanine and Cytosine, are held together by 3 Weak
Hydrogen Bonds
Look at the Base Pairs
Always A T, G C
DNA has Two Different Types of Bonds
• 1. Strong Covalent Bonds, which are found
between the Phosphate Groups and the
Nitrogenous Bases…. These strong bonds
prevent DNA from coming apart…. It’s bad if
your DNA falls apart.
• 2. Weak Hydrogen Bonds between the
Nitrogenous Base Pairs… these weak bonds
allow DNA to “Unzip” (come apart) when it
needs to self-replicate (make a copy of itself).
Replication
(The Process by which DNA makes a copy of itself)
When a Mother Cell divides, producing 2
Daughter Cells, each of the new daughter
cells must receive the information (DNA) it
will need to perform its life functions. As a
result of Replication, the Mother cell
copies its DNA and gives an exact copy to
each resulting Daughter cell. This ensures
that the Daughter cells will have the
genetic information (DNA) they need to
survive.
Replication occurs in the Nucleus.
(that’s where the DNA and necessary enzymes are)
Replication occurs during the “S” Phase of the
Cell Cycle.
In order to make DNA cells need nucleotides…where
do these nucleotides come from?
• Animals get Nucleotides by eating cells,
either plant or animal. The DNA is broken
down into nucleotides which animals can
then reassemble into DNA.
In Plants, compounds such as caffeine,
nicotine, rubber, amino acids, which are
needed for proteins, and nucleotides, which
are needed to make DNA and RNA are
produced during Respiration.
Respiration involves at least 50 different steps.
Each step in the break-up of sugars results
in different compounds.
The Process of Replication
Replication takes place in the 5’
to 3’ direction
Step 1. Helicase unzips the DNA
Step 2. DNA Polymerase
attaches DNA nucleotides to the
single DNA strand.
Step 3. DNA ligase glues the
DNA back together by forming
chemical bonds.