Download Nucleic Acids and Protein Synthesis

Nucleic Acids and Protein Synthesis:
DNA , RNA, and How Its Done
DNA Structure
• DNA Structure:
• Deoxyribonucleic acid
– Organic compound—nucleic acid
• Only two nucleic acids (RNA and DNA)
– Polymer—large molecule made of
repeating subunits
Building DNA Building Blocks
Nucleotide
• Nucleotide
– Made of three major parts
» Five carbon sugar
• DNA: Deoxyribose
• RNA: ribose
» Phosphate group
» Nitrogen base
• Four different kinds in DNA
• Adenine
• Guanine
• Thymine
• Cytosine
Complimentarity
• Pyrimidines bond to purines
– Adeninethymine (only)
– thymineadenine
– guaninecytosine (only)
– cytosineguanine
• specific bonding called complimentary base pairing
• creates a double helix
DNA Double Helix
DNA is Made of Two Long Chains of Nucleotides Joined by Hydrogen Bonds
A Nucleotide
G and C are
complementary as
are A and T
DNA Code
• Order of nitrogen bases determines the DNA
code
• A—T
• G—C
• A—T
• T—A
• G—C
• G—C
DNA Replication
Complementary Base Pairing Allows
Each Strand of DNA to Serve as a
Template for DNA Replication
DNA is a perfect illustration of
function following form
(structure dictates function).
DNA Replication: The process
• DNA Replication
• DNA replication: the process of copying DNA
in a cell
– The Replication Process
• DNA strands unwind and separate
– Separation point called the replication fork
– The strands are separated by enzymes called DNA
helicases
DNA Replication
DNA Replication: The Process (cont.)
• Another enzyme (DNA polymerase) binds to
separated chains and builds a new strand of
DNA using the complementary bases found in
the nucleus of the cell
–The fact that A only bonds with T and G
only bonds with C means the new strand
will be identical to the old separated strand
• Each new DNA molecule consists of one
old strand and one new strand
Accuracy and Repair
• Very accurate process
• One error in every 10,000 nucleotides
• One change of one nucleotide can cause big
problems—called mutation
• Special enzymes check the DNA for accuracy and
correct the mistakes which keeps the error rate to
1/1,000,000,000 nucleotides
• Some errors caused by damage to DNA from sunlight
or chemicals- called mutagens
Chapter 10
Section 4 Protein Synthesis
Comparing DNA and RNA
Click below to watch the Visual Concept.
Visual Concept
RNA: Ribonucleic Acid
• RNA Structure:
– Single stranded
– Ribose sugar in the
nucleotide
– Does NOT have thymine,
but instead has the
nitrogen base Uracil
– So: G—C and A—U (not A—
T; that is only DNA)
Types of RNA
– Messenger RNA (mRNA):
• Straight chain of RNA nucleotides
• Copied from the DNA template
• Carries information for protein synthesis to the cytoplasm
– Transfer RNA (tRNA):
• Bent, “hairpin” shaped molecule
• Assembles amino acids during protein synthesis
– Ribosomal RNA (rRNA):
• Globular form of RNA
• Makes up the ribosomes
• Site of protein synthesis
DNA Transcriptions
– the process by which genetic information is
copied from DNA to RNA
– mRNA serves as the messenger of the DNA code
to the cell
– mRNA is made from the DNA template and sent
from the nucleus to the ribosomes with the
instructions for making a protein
RNA Transcription
Steps to transcription
• 1. Begins with the enzyme RNA polymerase
• RNA polymerase binds to a special area of
the DNA called the promoter
– The promoter marks the beginning of the gene
– Only the gene involved in the specific protein
being made are transcribed into mRNA
– mRNA’s are short sequences copied from parts
of the DNA (called transcripts)
Steps to Transcription
2. The DNA separates (unzips) at the gene
• one of the strands will be transcribed—the
template strand
RNA Transcription
3. RNA polymerase attaches to the 1st
nucleotide in the gene sequence and begins
adding the complimentary base pairs to form
the RNA molecule
• the order of the nucleotides in the DNA
determines the sequence of the mRNA
– complimentary base pairing occurs the same as
in DNA EXCEPT
• A—U (the A on the DNA bonds to Uracil, not thymine;
but still G—C
RNA Transcription
4. Transcription continues until RNA
polymerase reaches a termination signal
• Marks the end of the gene
• 5. Piece of mRNA is released from the DNA
and the DNA “zips” back up
RNA Transcription
• 6. Piece of mRNA is moved through the
nuclear pores to the rough ER and finally the
ribosomes
– tRNA and rRNA are made in the same manner
Protein Synthesis
• The formation of proteins using the
information coded on DNA and carried out of
the nucleus by the mRNA
Protein Structure
– Made up of amino acids linked together into
chains called polypeptides
• 20 different amino acids occur in nature
• for a protein to function properly it must be made
correctly
• the structure is determined by the order, or sequence,
of amino acids in the polypeptides
– DNA holds the code that determines the order of amino
acids, and so, the function of the protein
Genetic Code
• Codons: 3-letter words
– Three sequential bases on the mRNA
•
• Ex.
• mRNA
A U C G U G C A C
– Each codon codes for one specific amino acid.
• Ex. AUC-- → Isoleucine
• Ex. GUG-- → Valine
• Ex. CAC-- → ______________
histidine
Protein Synthesis
What amino acids are coded for in the
following codons?
C G U
↓
Arginine
G G U
↓
Glycine
C A U
↓
Histidine
What message do you get from codons UAA
and UAG?
Translation
• Translation: reading the DNA to put together
the amino acids
– Occurs in the cytoplasm at the ribosomes
Steps to Translation
mRNA moves out of the nucleus to the
ribosome
• tRNA molecules transport amino acids
to the mRNA.
• Each tRNA has a specific amino acid
• Amino acids are free floating in the
cytoplasm
• 20 different kinds of tRNA
• anticodon: loop end of tRNA
• has sequence of bases
• complimentary to mRNA codon
Steps to Translation (cont)
• tRNA bonds to codon of mRNA with the
anticodon and the amino acids are lined up in
the correct order.
• Amino acids correctly arranged form peptide
bonds and are released as a polypeptide
when assembly is completed. Several
polypeptides may be needed for one protein.
Translation
Mutation
• Gene Mutation: a change in the DNA at a point
in the gene
•
• Can affect phenotype by changing the sequence
of DNA and, therefore, the resulting proteins.
•
• Ex. Codon CGA is replaced by CCA. The amino
acid in that chain would switch from arginine to
proline. That would change the function of the
protein.
Mutation: Point Mutations
• Types of gene mutation
• Point mutation: one base on the DNA
changed.
– Substitution: one base is substituted for another.
•
•
•
•
Ex.
CCU AAA UUU GGG GGC
Becomes:
CCU UAA UUU GGG GGC
Mutation
Frame Shift: Addition
• Frame Shift:
– Addition: a base is inserted into the sequence
•
•
•
•
Ex.
CCU AAA UUU GGG GGC
Becomes
CCU AAA AUU UGG GGG C…..
Frame Shift: Deletion
• Deletion: a base is deleted from the
sequence
• CCU AAA UUU GGG GGC
• Becomes
• CCU AAU UUG GGG GC…..
Mutation
Some mutation is good, too much is bad.
Cells employ elaborate mechanisms to prevent mutation – but the mechanisms
aren’t perfect.
Mutations are the root cause of cancer (bad).
Mutations are the only way to introduce novel alleles into a species (good for
evolution).
The effects of mutation are usually bad or neutral - only sometimes are
mutations beneficial.
So, just like Goldilocks – not to hot, not too cold, just right – the optimal rate of
new mutation is a balancing act.
Mutations
• All of the above can occur randomly or be caused by:
• Mutagens: environmental factors that cause
mutations.
•
•
•
•
•
•
Sunlight (ultraviolet radiation skin cancer)
Chemicals (asbestos, cigarette smoke)
Viruses
Radiation
Radon
Carcinogens (PCB’s)
DNA Damage is Often the Root Cause of Mutation
DNA is chemically altered (i.e. damaged) spontaneously and by chemicals and
radiation.
Mutation as Villain
Cancerous growths that
result from loss of a protein
that polices DNA for errors.
Cancer Incidence Increases Sharply with Age
The increase is due at least in part to the age-related accumulation of multiple
mutations in single cells.