DNA & Genes
Chapter 11
Discovery of DNA
 The composition of DNA was first
described correctly in 1952 by two
scientists:
1. Watson
2. Crick
 They discovered that it was formed from
two long chains of nucleotides shaped
much like a spiraling ladder.
 They called this shape the double helix.
DNA: The Molecule of Heredity
 Deoxyribonucleic Acid (DNA) is a very long
molecule that contains the information of life.
 DNA is a polymer made of repeating subunits
called nucleotides.
 A nucleotide has three parts:
 A simple sugar
 A phosphate group
 A nitrogen base
Structure of DNA
 Simple sugar
 The sugar in DNA is deoxyribose
 It gives DNA its name
 Phosphate group
 Composed of one atom of phosphorus
surrounded by four oxygen atoms
Structure of DNA
 Nitrogen Base
 A carbon ring structure that contains one
or more atoms of nitrogen
 There are four possible nitrogen bases:
 adenine (A)
 guanine (G)
 cytosine (C)
 thymine (T)
Nitrogen Bases
 Purine – double ring nitrogen bases
 Adenine (A)
 Guanine (G)
 Pyrimidine – smaller, single-ring
nitrogen bases
 Cytosine (C)
 Thymine (T)
Draw these structures in your notes – Page 288
Base-Pairing Rules
 The two sides of the double helix are
connected together by hydrogen bonds.
 Hydrogen bonds form between two
nitrogen bases that make up the rungs of
the ladder.
 The two nitrogen bases on the same rung
of the DNA ladder are referred to as a base
pair.
Base-Pairing Rules for DNA
 Guanine always pairs with Cytosine
 Adenine always pairs with Thymine
 The strictness of base-pairing results in two strands that
are complementary. This means the sequence of bases
on one strand determines the sequence of bases on the
other strand.
 Example:
T
C G A A T
T G C
__ __ __ __ __ __ __ __ __
Base-Pairing in DNA
 All organisms contain chromosomes composed of DNA
made up of nucleotides with guanine, cytosine, adenine,
and thymine.
 The reason organisms can be different from each other
even though their genetic material is made of the same
molecules is because the order of nucleotides is different.
 The sequence of nucleotides is what forms the genetic
information of an organism.
Base Pairing Review
A
T G C G T A T C
__ __ __ __ __ __ __ __ __
G
C T G A G C G T
__ __ __ __ __ __ __ __ __
T
A G C T C
G G A
__ __ __ __ __ __ __ __ __
C
G A C G T
C A G
__ __ __ __ __ __ __ __ __
Base-Pairing Review
1. How can two organisms be so different from
each other if their genetic material is made of the
same DNA --- G, C, A, T?
2. _____ and _____ are called purines because
they are a _______ ring of carbon and nitrogen.
3. _____ and _____ are called pyrimidines
because they are a _____ ring of carbon and
nitrogen.
The Central Dogma
 The central dogma describes how
information from DNA gets used to make
proteins.
 The Central Dogma involves three
processes:
1. Replication – copies DNA
2. Transcription – converts a DNA message into
RNA
3. Translation – interprets RNA into a string of
amino acids, called a polypeptide
DNA REPLICATION
 During mitosis and meiosis, the cells
divide. Each time a cell divides, it must
make a copy of its DNA.
 Replication is the process by which DNA is
duplicated, forming two identical copies
from one original.
3 Steps to DNA Replication
1. The enzyme helicase breaks the hydrogen bonds
between the nitrogen bases that hold the two strands
together – effectively unzipping the DNA molecule.
2. As the DNA continues to unzip, free nucleotides, from
the surrounding area in the nucleus, bond to the two
single strands according to the base pair rule.
3. The enzyme DNA polymerase forms the sugar-tophosphate bonds that connect the nucleotides on each
strand of DNA.
DNA REPLICATION
 Replication of DNA doesn’t begin at one
end of the molecule and proceed to the
other.
 It occurs simultaneously at many points on
the molecule, speeding up the process.
 Replication is completed when the entire
molecule has been unzipped and
replicated.
DNA REPLICATION VIDEO
https://www.youtube.com/watch?v=8kK2zwjRV0M
TRANSCRIPTION
 Transcription is the process of producing RNA
(Ribonucleic Acid) from DNA.
 RNA is the form of a nucleic acid in which
information moves from DNA in the nucleus to
the ribosomes in the cytoplasm.
 The process of transcription is similar to the
process of replication, with one exception:
 Adenine pairs with Uracil – not thymine
3 DIFFERENCES
BETWEEN DNA & RNA
1. RNA is single stranded – DNA is
double stranded
2. RNA’s sugar is ribose – DNA’s sugar is
deoxyribose
3. RNA has the base uracil – DNA has
the base thymine
3 TYPES OF RNA
1. mRNA – messenger RNA
 It carries the information from DNA (in the nucleus) out
into the cytoplasm.
2. tRNA – transfer RNA
 It brings amino acids to the ribosomes so they can be
assembled into proteins.
3. rRNA – ribosomal RNA
 It makes up the ribosomes, which are the site of
protein synthesis.
COMPARE & CONTRAST
REPLICATION & TRANSCRIPTION
 Similarities
 Both involve unwinding the DNA double helix
 Both involve large enzymes called polymerases
 Differences
 Replication makes an identical copy of DNA, but
transcription makes RNA molecules
 Replication happens only once during the cell cycle, but
transcription happens over and over on the same gene
to make many copies
Transcription Practice
1. GATACGATGACG
2. TACGTTACGCCA
3. GGTACAACGTGT
4. GATCGATCAAGC
5. GCAAACTTACGAG
6. CGATAGCATCGAT
TRANSLATION
 Translation is a process that converts a message from
one language into another, likewise, Translation happens
in your cells too.
 Cells translate an mRNA message into amino acids.
 A long strand of amino acids are known as a polypeptide,
which are the building blocks of proteins – this whole
process is called translation.
 An mRNA message can be translated into 20 different
amino acids.
TRANSLATION, CONTINUED
 The genetic code consists of four letters that make up 3letter “words” called codons.
 Each 3-letter codon codes for 1 of the 20 amino acids.
 Multiple codons can code for the same amino acid. For
example, the codons UCU, UCC, UCA, and UCG all
code for serine.
TRANSLATION, CONTINUED
 In addition to codons that code for amino acids, there is
a start codon & three stop codons.
 The start codon (AUG) signals the start of translation &
also codes for the amino acid methionine.
 The three stop codons (UAA, UAG, & UGA) signal the
end of an amino acid chain.
TRANSLATION, CONTINUED
 Why have a start codon anyway?
 For words to make sense, they must be read starting at the
correct place, or with the correct reading frame.
 For example:
 T HEC ATA TET HER AT
 TH ECA TAT ETH ERA T
 THE CAT ATE THE RAT
 There are no spaces between codons, but the correct
start site and the correct reading frame must be used for
the message to make sense.
TRANSLATION, CONTINUED
 The process of translation happens in the cytoplasm of
both prokaryotic and eukaryotic cells.
 The amino acid chain is bonded together with a peptide
bond.
 Transcription & Translation are key factors for protein
synthesis.
 DNA  RNA  Amino Acids  Protein
TRANSLATION PRACTICE
 AUGGUACUGUAA
 GCUAUGUGAUCU
 GUAUGCGUGGCGAGUAG
MUTATIONS
 A mutation is any mistake or change in the DNA
sequence.
 Mutations in gametes can be passed on to offspring of
the affected individual, but mutations in body cells affect
only the individual in which they occur.
 There are two types of mutations:
1. Chromosome Mutations
2. Gene Mutations
CHROMOSOME MUTATION
 A chromosome mutation involves a change in the
structure or number of chromosomes.
Explanation of chromosome mutations:
 Nondisjuction is when one or more pairs of homologous
chromosomes fail to separate during meiosis.
 Deletion – a piece of chromosome breaks off completely.
After cell division, the new cell will lack a certain set of
genes. Often this is fatal to the zygote.

CHANGES IN
CHROMOSOME STRUCTURE
 Insertion (Duplication) – A chromosome fragment attaches
to its homologous chromosome, which then carry two
copies of a certain set of genes
 Inversion – the chromosome piece reattaches to the original
chromosome but in a reverse position
 Translocation – the piece reattaches to a nonhomologous
chromosome
GENE MUTATION
 Gene mutation involves a change in the chemical
makeup of the DNA.
 Point mutations occur when one or more DNA
nucleotides are deleted or substituted with others.
 Frameshift are mutations that change one or just a few
nucleotides in a gene on a chromosome
 Sickle cell anemia
 Cystic Fibrosis
MUTATIONS
 Occasionally random gene mutations produce changes
that make the individual better adapted to the
environment. Such mutated genes tend to increase in
frequency within a population
MUTAGENS
 Mutagens are factors that increase the number of
occurrences of mutations.
Examples:
 Ultraviolet Rays
 Radioactive Substances
 Chemicals
 Nicotine
 Alcohol