Download DNA and Protein Synthesis

DNA and Protein
Synthesis
Biology 12
Chapter 24
Overview/Objectives
• Part 1: DNA
I. DNA structure
II. Functions of DNA
III. DNA Replication
• Part 2: Protein Synthesis (Gene Expression)
I. RNA
II. Protein Synthesis
a.
b.
Transcription
Translation
III. Mutations
Part 1: DNA
DNA = Deoxyribonucleic Acid
= The genetic material of
all living things
= Contains all of the
“instructions” needed
for life
I. DNA structure
• Determined by Watson and Crick in the early
1950’s
• Made up of nucleotides  each nucleotide
has 3 subunits:
o Phosphate group
o Pentose sugar (deoxyribose)
o Nitrogen-containing base
• 4 possible bases:
Purines (Double Ring)
Pyrimidines (Single Ring)
Adenine (A)
pairs with 
Thymine (T)
Guanine (G)
pairs with 
Cytosine (C)
Complementary
base pairing
Done by
hydrogen bonds
• DNA is a double stranded molecule
o Strands are held together by hydrogen bonding
o Alternate Sugar-Phosphate backbone
o Strands twist to form a double helix
o Two strands run antiparallel (in opposite directions)
II. Functions of DNA
1. Store information: control cell activity via.
Genes
2. Have mutations: allows for evolution
3. Replicate: make copies of self before cell
division to ensure daughter cells get equal
amount of DNA
III. DNA Replication
• Needed when a cell divides
 each new cell needs a copy
of the DNA
• Semi-conservative replication
 each new double helix has
one old strand and one new
strand
Steps to DNA Replication
1. Enzyme DNA helicase unwinds and unzips
the double-stranded DNA, breaking the
hydrogen bonds
2. New nucleotides base-pair to parent strands
helped out by enzyme DNA polymerase
3. Enzyme DNA ligase seals the sugarphosphate backbone
4. 2 double helix molecules identical to each
other, having one parent and one new strand
Part 2: Protein Synthesis
(Gene Expression)
• Gene: A segment of DNA that specifies the
amino acid sequence of a protein
• Genes pass their info to RNA  RNA is
involved in protein synthesis
I. RNA
• RNA = Ribonucleic Acid
• Type of nucleic acid consisting of:
o Ribose sugar
o Phosphate
o Base: adenine, guanine, uracil, cytosine
o Single stranded
RNA
vs.
DNA
Sugar
Ribose
Deoxyribose
Bases
AGUC
AGTC
Strand
single
double
Helix
no
yes
Types of RNA
3 types:
• Messenger RNA (mRNA): Takes a message
from DNA to ribose
• Ribosomal RNA (rRNA): Along with proteins,
make up the ribosomes (location of protein
synthesis)
• Transfer RNA (tRNA): Transfers amino acids to
the ribosome
II. Protein Synthesis
• Two steps involved
with protein synthesis
a) Transcription 
genetic information is
transferred from DNA to
RNA
b) Translation  RNA
transcript directs the
sequence of amino acids
in the polypeptide
(protein)
a) Transcription (step 1)
• Process of creating mRNA from a DNA template
1. Enzyme RNA polymerase binds to a promoter
(special nucleotide sequence) on DNA, and
opens up the DNA helix
a) Transcription (step 2)
2. Complementary base pairing occurs and RNA
polymerase joins the RNA nucleotides,
forming an RNA molecule
a) Transcription (step 3)
3. Processing of RNA
I.
One end is “capped”(addition of altered guanine
nucleotide)
Addition of poly-A-tail to the other end
II. Introns removed, exons joined
a) Transcription (step 4)
4. Mature mRNA leaves the nucleus through a
nuclear pore and enters cytoplasm
The Genetic Code
• Triplet code  every 3 bases of mRNA codes
for 1 amino acid
– Each 3 letter (nucleotide) unit of mRNA is called a
codon
• tRNA brings amino acids to ribosomes: one
end holds the amino acid, the other end
contains an anticodon  a group of 3 bases
complementary to a codon on mRNA
Example: Reading the Genetic Code
Which amino acid sequence is formed from the
DNA strand:
GGT ACA
TAG
CAC
DNA
CCA UGU
AUG
GUG mRNA
proline-cysteine-isoleucine-valine amino acids
Use table 24.8 p.492
b) Translation
• The step by which gene expression leads to
protein synthesis
• 3 steps:
1. Chain initiation
2. Chain elongation
3. Chain termination
b) Translation (step 1)
Chain Initiation
• Smaller ribosomal subunit binds mRNA
• Initiator tRNA binds
• Larger ribosomal subunit binds, completing
the ribosome
b) Translation (step 2)
Chain Elongation
• tRNA binds in 2nd binding site of ribosome
• Peptide bond forms between amino acids
• tRNA in 1st binding site leaves, ribosome moves
down mRNA (translocation)
• tRNA in 2nd binding site has moved to 1st binding
site… cycle continues…
b) Translation (step 3)
Chain Termination
• Stop codon on mRNA reached
• Polypeptide (amino acid chain) is cleaved from
the last tRNA
• Ribosome separates and releases mRNA
III. Mutations
Mutation= any alteration in the code or expression of
the code (DNA)
Causes of Mutations:
• Errors in Replication
• Mutagens (environmental influences)
• Transposons (DNA sequences that move between
chromosomes)
Types of Mutations:
• Frame shift mutation (nucleotides are inserted or
deleted)
• Point mutation (substitution of one nucleotide for
another)