The DNA Molecule
Chapter 12
DNA
• DNA
– Deoxyribonucleic Acid
– Molecules that make up genes, determines traits
of all living things
– DNA is in every (nucleus) cell of your body
• Models of DNA
– A ladder can be used to represent DNA
– Human DNA can contain 100 million rungs
• There are 5 features to DNA
1. DNA has two sides
2. Sides made of two chemicals
1. One part sugar (Deoxyribose)
2. One part phosphate
Sugar
Phosphate
3. Connecting the two sides are the nitrogen
bases
4. Four different nitrogen bases
1. Adenine: A
2. Thymine: T
adenine
3. Cytosine: C
4. Guanine: G
thymine guanine
cytosine
5. Bases have exact fit to form the rungs of
the ladder.
1. A - T.
2. C - G.
Shape of DNA
•DNA is a double helix where two strands are
wound around each other.
Nucleotide
•Each strand is made of Hydrogen
bonds
nucleotide chain.
•Nucleotide = base +
phosphate + sugar
•Strands are held
together by hydrogen
bonds between A and T
and G and C.
Chargaff’s rules:
• DNA samples have almost equal amount of
A as T and C as G.
• Why?
• For every A there is a T, for every C there is
a G because A bonds with T and C bonds
with G.
DNA replication
• process by which DNA is
copied
• When occur?
• occurs in S phase of cell
cycle
• enzymes separate the two
strands
• new (complimentary) strands
made that match the bases
left over
RNA and protein synthesis
• RNA
• ribonucleic acid
• carries out the
instructions from
the DNA code
Differences between DNA and
RNA
• sugar in DNA is deoxyribose
• sugar in RNA is ribose
• DNA is double-stranded
• RNA is single stranded
• DNA has A, T, C, G
• RNA has A, U, C, G ( uracil-U
replaces T)
• In RNA, U bonds to A and C
bonds to G
Forms of RNA
• 1. Messenger RNA - mRNA
- serves as messenger from
DNA to the rest of cell
- carries instructions for
building of proteins
2. Ribosomal RNA – rRNA
becomes part of the ribosomes, where
proteins are made
3. Transfer RNA – tRNA.
transfers amino acids from
cytoplasm to the
ribosome.
amino acids are assembled
into polypeptides.
polypeptides are assembled
into proteins.
Transcription
• process that makes RNA
• a section of DNA is copied into RNA
• an enzyme (RNA polymerase) binds to a DNA
molecule. It produces a strand of RNA, one
nucleotide at a time
Transcription
• DNA RNA copy
» A U
» T A
» C G
» G C
Benefits of RNA
• DNA can be copied into RNA repeatedly
• many copies made for rapidly dividing cells
• DNA not leave nucleus, it is kept safe
• Purpose of RNA
• to make proteins
Translation - decoding of a mRNA message into a
polypeptide chain
•mRNA codes for amino acids
•amino acids join together to form polypeptides
•polypeptides join together to form proteins
mRNA
tRNA
rRNA
A three-nucleotide
sequence of mRNA
codes for an amino acid
is a codon.
Codons
Examples of codons: AUG CAU UCU
AUG is the code for Methionine
CAU is the code for Histidine
UCU is the code for Serine
Page 303
Methionine, Histidine, and Serine are amino acids. These
amino acids are translated from the genetic code.
Mutations - changes in the
DNA
DNA sequence that affect
genetic information
RNA
Protein
No color
in the
flower
Two major types:
1. gene mutations- changes affecting a
single gene
gene
2. chromosomal mutations- changes in
whole chromosomes
chromosomes
Types of gene mutations:
1. Point mutation
- substitution of one nucleotide for another
- affects only one codon
Normal
AGG TCT GAT TAA
Point Mutation
AGG TCT GTT TAA
Types of gene mutations:
2. frameshift mutation
- insertion or deletion of a nucleotide
- affects many nucleotides and codons
Normal
AGG TCT GAT TAA
Frameshift Mutation
AGG CTG ATT AAT
How can the rate of mutations be increased?
- radiation (x-rays, ultraviolet rays, etc.)
- certain chemicals
Now YOU are all DNA & RNA GENIUSES
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