CHAPTER 12
Hershey & Chase: Discovered evidence that DNA is
genetic material NOT protein
DNA STRUCTURE: Deoxyribonucleic Acid
*Long molecules made up of repeating subunits called
nucleotides
*Double Helix Structure
*Nucleotides have 3 parts:
1. Simple Sugar: Deoxyribose
2. Phosphate group: 1 atom P surrounded by 4 O2
3. Nitrogenous base: Carbon ring structure contains
1 or more atom of N.
4 NITROGENEOUS BASES: 4 possible nucleotides
containing:
1. Adenine (A)
2. Guanine (G)
3. Cytosine (C)
4. Thymine (T)
NUCLEOTIDES: Join together to form long chains with
the phosphate group of 1 nucleotide bonding to the
deoxyribose sugar of an adjacent nucleotide.
WATSON & CRICK: Proposed that DNA is made of 2
chains of nucleotides held together by nitrogenous
bases held together in the form of 2 strands zipping
together. The 2 strands of DNA have weak hydrogen
bonds that can only form between certain bases.
*Adenine pairs only with Thymine
*Guanine pairs only with Cytosine
EXAMPLE: A-T-T-G-C (Equals the base strand)
T-A-A-C-G (Equals the complementary strand)
??HOW CAN ORGANISMS BE SO DIFFERENT IF THEIR
GENETIC MATERIAL IS MADE OF THE SAME 4
NUCLEOTIDES??
*Because the sequence of the 4 different
nucleotides varies along every DNA strand.
EXAMPLE: A-T-T-G-A-C carries different info than
T-C-C-G-A-A. The closer the relationship
between 2 organisms, the more similar the DNA
nucleotide sequence will be.
DNA REPLICATION: DNA in the chromosomes is copied.
Without DNA replication, new cells would have only ½
the DNA of their parent cells.
HOW IT REPLICATES: Each base strand serves as a
pattern or template for the complementary strand to
make the DNA molecule. There are 4 steps in
replication:
1. Replication begins as an enzyme that breaks
the hydrogen bond between bases that hold 2
strands together; unzipping the DNA & exposing
the bases. DNA is copied during interphase prior
to mitosis and meiosis.
2. Bases in free nucleotides pair with exposed
bases in DNA Strand (in their assigned pairs)
3. Bonding of bases: Adjacent nucleotides bond
together to form the backbone of the new strand
4. Results of Replication: Produces 2 molecules of
DNA. 1 strand from original molecule and 1 from
newly synthesized free nucleotides in a cell.
DNA TO PROTEIN
Information in DNA nucleotides are used for
production of proteins. Proteins are used to perform
KEY life functions (digestion, cell formation, etc)
RNA: A nucleic acid that differs in structure from DNA
in 3 ways:
RNA
DNA
1. Single stranded
Double stranded
2. Sugar=Ribose
Sugar=deoxyribose
3. Thymine replaced
Thymine
by Uracil
3 TYPES OF RNA:
1. mRNA/Messenger RNA: Brings instructions
from DNA in the nucleus to ribosomes in the
cytoplasm
2. rRNA/Ribosomal RNA: Binds to mRNA and uses
the instructions from DNA to assemble Amino
Acids
3. tRNA/Transfer RNA: Delivers the assembled
Amino Acids to Ribosomes to be assembled into
proteins
TRANSCRIPTION: Similar to DNA replication but the
end result is the formation of 1 single-strand RNA
molecule. Enzymes break Hydrogen bonds between
nucleotide pairs. Free nucleotides pair with base
strand, bonding to form new strands.
GENETIC CODE: Nucleotide sequence is transcribed
from DNA to mRNA to form a genetic message. Each
sequence acts as a written language using nitrogenous
bases (A-T-C-G) as its own alphabet.
Biochemist cracked the code when they discovered
that a group of 3 nitrogenous bases in mRNA code for
1 amino acid. This is called a CODON.
GENETIC MUTATIONS
Mutation- any permanent change in the DNA
sequence. Can occur in reproductive & somatic
(body)cells.
Mutations are caused by errors in:
1. Replication
2. Transcription
3. Cell Division
4. External Agents
Mutations in Reproductive cells are due to a change in
the nucleotide sequence in sperm or egg and can
result in: (may have positive or negative effects)
1. A new trait
2. Protein working incorrectly
3. Structural or Functional problems
4. Non-functional protein (embryo does not live)
Mutations in body cells affect genes that control cell
division. Ex: Cancer-results from UV radiation.
TYPES OF MUTATIONS:
-Point Mutation: Change in a single base pair (incorrect
amino acid inserted)
-Frameshift Mutation: Single base is added or deleted
-Chromosomal Mutation: Parts can join to wrong
chromosomes or in the wrong direction/backwards
CAUSES OF MUTATIONS:
-Mistakes in base pairings during DNA replication
(spontaneous reaction)
-Environmental Factors: Example: MUTAGEN-an agent
that can cause a change in DNA.
EX: X-Ray, UV Rays, Nuclear Radiation, Chemicals
(dioxides, asbestos, formaldehyde)