DNA: The
Molecule of
Heredity
DNA – The Molecule of
Heredity
A. What is DNA?
1. Deoxyribonucleic Acid
2. Has two major functions:
a. Copy itself exactly for new cells to be created
1. For example: every human needs an exact copy of their parents’ combined DNA
2. Hereditary material of the cell
b. Stores and uses information to direct activities of the cell
1. Controls the production of proteins within the cell
DNA – The Molecule of
Heredity
3. Where is it found?
Eukaryotic Cell
Prokaryotic Cell
Nucleus
Cell Cell Membrane Chromatin (DNA)
DNA – The Molecule of
Heredity
B. Structure of DNA?
1. Polymer
(poly = many)
a. Composed of repeating monomers (mono = one)
b. The repeating units are called nucleotides
2. Nucleotides
a. Have 3 parts:
1. 5 carbon sugar called Deoxyribose 2. Phosphate group
3. Nitrogen base
DNA – The Molecule of
Heredity
c. There are 4 different kinds of nitrogen bases
that differ structurally:
1. Adenine (A) 2. Guanine (G) 3. Thymine (T) 4. Cytosine (C)
d. These 4 bases are grouped into 2 broader
categories based on their shape
1. Purines – adenine & guanine a. Larger molecules
DNA – The Molecule of
Heredity
3. Double helix
a. Model of DNA where 2 strands twist around a central axis to form a spiral structure
b. James Watson & Francis Crick worked together in England during the early 1950s
1. They deduced the structure of DNA was a double helix
DNA – The Molecule of
Heredity
4. How to form the double helix
a. Have a string of phosphate, sugar,
phosphate, sugar … (outside of “ladder”)
b. Have nitrogen bases on the “rung” of the
ladder
1. Nitrogen bases are held together by hydrogen bonding – share hydrogens
2. Purines always bond with pyrimidines
a. Cytosine – Guanine (C-G)
1. Shares 3 hydrogens – stronger bonds
b. Thymine – Adenine (A-T)
DNA – The Molecule of
Heredity
c. Strands are said to be “
complementary
” to one
another because they “fit” together
1. The structures of the bases literally fit together like a puzzle
A
T
T
A
C
G
DNA – The Molecule of
Heredity
C. DNA Replication
1. The process of making an exact copy of the DNA
2. Needed when a cell divides so the new cell can have
an exact copy of the DNA
3. The whole process is controlled by
enzymes
4. Steps:
a. DNA Helicase (enzyme) attaches itself to the DNA and unwinds it, “unzips” it
1. This makes 2 single strands of DNA
DNA – The Molecule of
Heredity
b. DNA Polymerase (enzyme) enters
1. There are free bases (A, T, C, G) floating around in the nucleus
2. DNA polymerase takes these bases and attaches them to their compliment on each of the DNA strands
3. The original DNA strand serves as a “template” to add bases and create a new strand
c. DNA Ligase
1. These enzymes stitch together the new DNA fragments until 1 continuous strand is made
5. Process moves away from the 3’ end of the parental
DNA strand toward the 5’ end
a. The 3’ end – free hydroxyl group of sugar b. The 5’ end – free phosphate group
DNA – The Molecule of
Heredity
6. Process occurs at several places on a DNA
strand or else it
would take too long
DNA – The Molecule of
Heredity
D. Accuracy and Repair
1. The cell has a
built-in “proofreading” function to
make sure replication is accurate and correct
2. There is an error on average for every 1 billion
nucleotide added –
point mutations
a. Some errors can be caused by environmental agents – UV light, nicotine, etc.
3. There are repair enzymes that remove the damaged
nucleotide and replace them with the correct one
4. The sequence of bases is
not random
!
a. Inherent in the sequence is all the information needed for determining the structure and function of an organism b. DNA is just the first part of a whole process that will make
DNA – The Molecule of
Heredity
Part 2 – Gene
Expression &
Regulation
A.RNA
1. Polymer, consisting of monomers –
nucleotides
2. RNA differs from DNA in 3 ways:
RNA DNA
1. Single stranded 1. double stranded
2. Ribose sugar 2. deoxyribose sugar
3. Nucleotide base – uracil (U) 3. nucleotide base
thymine (T)
Gene Expression &
Regulation
3. Codon
a. A sequence of 3 bases – a triplet
code
4. 3 Types of RNA:
a. Messenger RNA (mRNA) (the
message)
1. Single, uncoiled strand that
transmits information from DNA,
made up of bases
Gene Expression &
Regulation
b. Transfer RNA (tRNA) (transferring the
message)
1. Single stranded, folded
2. Contains an anticodon region that binds with codon of mRNA
3. Bonded to an amino acid on the other end 4. 20+ varieties
5. Each bond to only 1 specific type of amino acid
Amino Acid
Anticodon G A C G A C
Amino Acid Anticodon
G A C
Gene Expression &
Regulation
c. Ribosomal RNA (rRNA)
1. Globular form of RNA
2. Makes up ribosomes
3. Has binding sites for mRNA and tRNA
Amino Acid Anticodon
G A C
Gene Expression &
Regulation
B. Process of Transcription
1. Process where RNA is produced from DNA
a. RNA serves as the “bridge” between DNA
and proteins
2. RNA molecules are made from the
information encoded in the base sequence
of DNA
Gene Expression &
Regulation
3. Steps of Transcription:
a. RNA polymerase binds to DNA, unwinds
it, unzips it at the “promoter”
1. Only one strand is needed for
transcription, only 1 serves as the
template strand
2. Promoter
Gene Expression &
Regulation
a. TATA box, a nucleotide sequence
containing TATA about 25 nucleotides away
from the transitional start point
CGTCTATAAAACGTTGCT………start
point
GCAGATATTTTGCAACGA………
Gene Expression &
Regulation
b. The template strand is copied just like in DNA
replication, with its complimentary base being
matched up to the template strand
1. Only difference – uracil is bound to adenine (U-A) 2. There are no thymines (T) in RNA
c. When RNA polymerase reaches the “stop
codon,” it falls off and DNA winds up again
d. 60 nucleotides/second in eukaryotes
Gene Expression &
Regulation
f. RESULT:
1. Created an RNA strand with bases that
are complimentary to the DNA template
2. All the information that was in DNA is
Gene Expression &
Regulation
4. The newly formed RNA strand
contains fragments coding for an
amino acid sequence – exons and
other noncoding segments – introns
Gene Expression &
Regulation
C. Process of Translation
1. The process of making a protein
molecule from the information found in
mRNA
2. What is a protein?
Gene Expression &
Regulation
b. Every cell in your body contains as many
as 2000 different proteins
c. Proteins make up parts of your cell and
help your cell function
d. Protein help build your muscles up
e. Proteins help your blood clot so you don’t
bleed all the time
Gene Expression &
Regulation
3. Steps of Translation:
a. mRNA moves to a group of ribosomes b. tRNA carries its amino acid to the mRNA
c. The anticodon region on the tRNA lines up with its compliment on the mRNA strand
d. Another tRNA with the next complimentary
anticodon will come along and line up next to the first tRNA
e. The two amino acids, one from the first tRNA and the other from the second tRNA, will bind together f. The first tRNA will fall off
Gene Expression &
Regulation
D. Point mutations
1. Mutation – change in the genetic material of the cell 2. Point mutation – change in just one base pair of a
gene
a. Can be transmitted to future generations through reproduction
b. If the mutant gene has an adverse effect – called a genetic disorder
3. Some chemical mutagens can cause cancer
a. X-rays and other forms of high-energy radiation pose hazards to genetic material
b. Example: recent tsunami in Japan with nuclear explosion!
