Download The Genetic Code

Mrs. Williams
Freshman Biology
Semester Two
Discovery

Where does our inheritance come from?
 Thought to be either DNA or protein
 Several experiments were performed by
various scientist
 Conclusion: DNA is the molecule that
provides heredity

What does DNA look like?
Rosalind Franklin (1951)
Chargaff
Noticed that the percentage of adenine and
thymine were similar in DNA samples
 Also, the percentages of cytosine and
guanine were similar

Watson & Crick (1953)


Won the Nobel Prize
for determining the
structure of DNA
Proposed that it was
a double helix with
bases pairing in the
middle (like a
twisted ladder)
Nucleic Acid: Basic Structure
Nucleotidemonomer of a
nucleic acid
(polymer)
 Three parts

Phosphate
Sugar
Nitrogen Base
 Phosphate Group
 Sugar
 Nitrogenous Base
DNA Nucleotide
= Phosphate
PURINES
= Deoxyribose
PYRIMIDINES
= Adenine
= Thymine
= Guanine
= Cytosine
DNA

Pyrimidine
 Single ring structure; C and T

Purine
 Double ring structure; A and G

Base pairing
 One purine and one pyrimidine
A=T
G=C
A
T
C
DNA Structure

Sugar-Phosphate
Backbone
 Double stranded
 Alternating
 Inverted strands
 Base attached to sugar

Base Pairing
 A pairs with T
 C pairs with G
 Weak Hydrogen bonds
hold them together
Nucleic Acids
Location of DNA

Prokaryotes
 Floating in cytoplasm
 Occurs as a ring

Eukaryote
 Located in Nucleus
 Chromatin- form present during interphase
 Chromosomes- form present during
mitosis/meiosis
Chromatin and Chromosome
Structure

Chromatin
 DNA is coiled around
histone proteins
 Looks like a beaded
necklace
 Provides access to genes
during interphase

Chromosome
 DNA is super-coiled
 Compacts the DNA for
more efficient movement
 Less chance of damage
or mistakes
DNA Replication





DNA
Formation of a new DNA molecule
Occurs in the nucleus during S phase of
interphase
Goal- to create a copy of every piece of DNA
before cell division
Semi-conservative
 Each original strand serves as a template
 Ending DNA molecules have one original strand and one
new strand
Process of DNA Replication

DNA is unzipped
 Done by enzyme (helicase)
 Creates replication forks
 Many replication forks along length of DNA
strand
Process of DNA Replication
(continued)

Synthesis of new DNA strands
 Done by enzyme (DNA polymerase)
 Occurs in opposite direction on the two strands
 Bases are added according to base pairing rules
(A-T and C-G)
Process of DNA Replication
(continued)

Finishing Touches
 Backbone is sealed (done by enzyme: ligase)
 Proofread (done by enzyme: DNA polymerase)
RNA: The Other Nucleic Acid
RNA
Found in both the nucleus and the
cytoplasm
 Also composed of nucleotides
 Functions to


 Turn DNA instructions into a protein
 Regulate gene function

Differences from DNA
 Sugar is ribose instead of deoxyribose
 Thymine is replaced with uracil
 Single strand (backbone)
RNA NUCLEOTIDE
= Phosphate
Purines
= Ribose
Pyrimidines
= Adenine
= Uracil
= Guanine
= Cytosine
RNA STRUCTURE
•Single-stranded
Types of RNA


All are made as copies of the DNA
Messenger RNA (mRNA)
 Single strand forms a string
 Carries DNA instructions for a protein to the
ribosome

Transfer RNA (tRNA)
 Single strand folds into clover leaf shape
 Carries amino acids to build the protein to the
ribosome

Ribosomal RNA (rRNA)
 Single strand folds into 3D shape
 Creates ribosome structure
Protein Synthesis
DNA
RNA
Protein
Transcription

DNA gene is transcribed (copied) into mRNA
 mRNA bases are added by complementary base
pairing rules


Occurs in nucleus
Done by enzyme (RNA polymerase)
 Separates DNA strands
 Uses one strand as template to base pair
 When finished, mRNA breaks off and DNA binds
together again



mRNA is processed
Leaves the nucleus through pores
Travels to ribosome in cytoplasm
Complementary Base Pairing
DNA Nitrogen
Bases
A=U
T=A
C=G
G=C
RNA Nitrogen
Bases
Translation





mRNA is translated (decoded) into a
protein molecule at ribosome in cytoplasm
mRNA instructions are read three bases at
a time- codon
Every codon matches with a tRNA
anticodon
tRNA is attached to a specific amino acid
(protein monomer)
Amino acids are joined at the ribosome to
form a protein
Genetic Code





Every codon codes
for a specific amino
acid
This is called the
genetic code
64 possible codonsonly 20 amino acids
Start codon- AUG
Stop codonsUGA, UAA, UAG
Mutations
Change to the genetic material
 Mutations within a gene

 Point Mutation- occurs at one point in the DNA
○ Insertion- extra base added to gene
○ Deletion- base removed from gene
○ Substitution- one base is exchanged with another
 Frameshift Mutation- moves all remaining bases
forward or backward; changes all of the codons
after it (insertion and deletion)

Mutations
Chromosome Mutation





Change in genetic material that can be
seen on a chromosomal level
Duplication- part of a chromosome has
been repeated
Deletion- part of a chromosome has been
lost
Inversion- part of a chromosome has been
flipped
Translocation- part of a chromosome has
broken off and attached to another
chromosome
Consequences of Mutations

Most don’t do anything
 Genetic change causes no change to protein
 Protein change causes no change to function

Some are harmful
 Protein change causes loss of function or gain
of a new unwanted function

Some are beneficial
 Protein change allows protein to work better or
gain a new wanted function
Gene Regulation
All somatic cells of an individual contain
the same DNA
 Different cell types just use different
parts of the DNA library
 Since these cells use different proteins,
they look and act differently (cell
specialization or differentiation)
