Download Nucleic Acids and Protein Synthesis

Nucleic Acids and Protein
Synthesis
Chapter 7
DNA
7-1
The Genetic Code
• What is the structure that carries genetic
information?
• Scientists wondered this for years, and through
many experiments, they figured out exactly what it
was.
Griffith and Transformation
• Frederick Griffith wanted to know how bacteria
made people sick.
• He isolated 2 separate strains of pneumonia
bacteria from mice.
– Only 1 of these caused pneumonia
– The disease-causing strain of bacteria grew into smooth
colonies (S-bacteria)
– The harmless strain produced colonies with rough edges
(R-bacteria)
• The Experiment
– Griffith injected mice with S-bacteria.
• Mice died from pneumonia
– Mice injected with R-bacteria
• Mice never got sick
– Heat-killed S-bacteria and then injected it into mice
• Mice survived.
– Mixed heat-killed S-bacteria with R-bacteria and injected them into mice.
• Mice got sick and died.
– The dead bacteria somehow passed on its diseasecausing ability to the harmless bacteria.
– Transformation: The process by which one strain of
bacteria is changed by a gene or genes from another
strain of bacteria.
Avery and DNA
• Decided to repeat Griffith’s experiments, but also
wanted to figure out which part of the bacteria was
responsible for the transformation.
• They had it down to being a protein, carbohydrate, or
DNA.
• Made a juice from heat-killed S-bacteria.
• In different experiments, they introduced enzymes that
destroyed proteins, carbohydrates, and DNA.
• When the proteins and carbohydrates were destroyed,
transformation occurred.
• When DNA was destroyed, transformation did not
occur
– DNA is the structure that passes on genetic information.
Hershey-Chase Experiment
• Scientists still doubted that DNA contained the
genetic info. It was too simple.
– They thought it was protein.
• Hershey and Chase worked with viruses, specifically
bacteriophages.
– Bacteriophage: A virus that infects bacteria
• A bacteriophage is made of only 2 things: DNA and Protein
• To figure out which component was passed on, they tagged the
viruses with phosphorus-32 and sulfur-35.
– Proteins don’t contain phosphorus
– DNA doesn’t contain sulfur.
• If P32 was found in the bacteria, it’s the DNA that is transferred. If
S35, it’s the protein.
• Results: P32 was found in the bacteria.
– DNA is definitely the genetic material.
The Structure of DNA
• What is the basic unit of a nucleic acid?
– A nucleotide.
• Made of 3 parts:
– 5-carbon sugar (deoxyribose)-Side of ladder
– Phosphate Group- Side of ladder
– Nitrogenous Base
» There are 4 bases
• Adenine (A), Cytosine (C), Guanine (G), Thymine (T)
• These are placed into 2 groups:
• Purines: 2 rings in the structure.
• Adenine, Guanine
• Pyrimidines: 1 ring in structure
• Cytosine, Thymine
Chargaff’s Rules
• Discovered that in DNA samples, percentages of
guanine and cytosine are almost equal to one
another. Adenine and Thymine percentages were
also similar.
[A]=[T], [C]=[G]
X-Ray Diffraction
• Rosalind Franklin used x-ray diffraction to get a
general shape of the DNA molecule.
– Aimed a powerful x-ray beam at concentrated DNA
samples.
• Results:
– DNA is spring-shaped (helix)
– Since it looked like an x, there are 2 helices (Double Helix)
– Nitrogenous bases are in the middle
Making the Model
• Watson and Crick used Chargaff’s rules and Franklin’s images to
create a 3-D model.
• It resembled a spiral staircase with nitrogenous bases in the middle.
• Base-Pairing: Adenine goes with Thymine, Guanine goes with
Cytosine.
• Held together with hydrogen bonds
– 2 bonds between A and T
– 3 bonds between G and C
DNA and Chromosomes
• DNA length
– The DNA in a single human cell measures out to
between 1-2 meters.
– The total length of all the DNA in a human is 2.0 x 1013
meters.
• That’s 20,000,000,000,000 meters
– At that distance, you would be able to travel to the sun and back
almost 70 times.
» Distance from Earth to the Sun = 93,000,000 miles.
• Chromosome Structure
– Chromosomes are made of both DNA and proteins.
– Chromatin: Consists of DNA tightly wound around
proteins.
• Histones: The proteins
– When the DNA and several histones are wound together,
a nucleosome is created.
• Nucleosomes wind together to create chromosomes.
DNA Replication
• Each strand of DNA can be used as a template to
create another strand.
– The strands are complimentary.
– If you only had one strand, you could figure out the base
sequence to the other side.
• Practice: Construct the complimentary strand to the following
base sequence:
ACCGATATGCAAT
• In prokaryotes, DNA replication begins as a single
point and then moves in both directions until
replication is complete.
• In Eukaryotes, replication occurs in hundreds of
places on a single strand.
• Replication Fork: The sites where separation and
replication occur.
– Duplicating DNA
• Replication: The process of copying DNA
• During DNA replication, the DNA molecule
separates into 2 strands, and then produces 2 new
complimentary strands following the rules of base
pairing. Each strand of the double helix of DNA
serves as a template, or model, for the new strand.
• How DNA Replication occurs
– DNA Helicase (an enzyme) “unzips” the DNA molecule.
• Done by breaking the hydrogen bonds between the nitrogenous bases.
– DNA Polymerase joins individual nucleotides to produce a DNA molecule.
• Also, it proofreads each DNA strand to eliminate mistakes.
– DNA Ligase binds the DNA strand back together.
RNA and Protein
Synthesis
7-2, 7-3
• Gene: Coded DNA instructions that control
the production of proteins within the cell.
– Well how are proteins made from the genes?
• Protein Synthesis is divided into 2 stages.
– Transcription
– Translation
• The first step (transcription) is to copy part of
the nucleotide sequence from DNA into RNA,
which then makes the proteins
• RNA= ribonucleic acid
– The assembly of amino acids into proteins is
controlled by RNA
The Structure of RNA
• Very similar to DNA structure, but 3 differences.
– The sugar is Ribose
– RNA is usually single-stranded
– Instead of having Thymine, it has Uracil
Types of RNA
• 3 main types
– mRNA
– rRNA
– tRNA
• mRNA= messenger RNA
– Copies the instructions from genes in DNA.
– Serves as the messenger from the DNA to the rest of the cell.
• rRNA= ribosomal RNA
– Proteins are synthesized/assembled by ribosomes
– Ribosomes are made of rRNA and other proteins.
• tRNA= transfer RNA
– Transfers amino acids to the ribosome to be assembled into
proteins.
Transcription
• Transcription: Process of copying part of the
nucleotide sequence of DNA into a
complimentary sequence of RNA.
• Occurs in the nucleus.
• Works very similarly to DNA replication.
– RNA Polymerase works a lot like DNA Polymerase
• RNA polymerase binds to DNA and separates
the DNA strands. RNA polymerase then uses 1
strand of DNA as a template from which
nucleotides are assembled into strands of
RNA.
• How does the RNA polymerase know
where to start and stop copying?
– RNA polymerase doesn’t bind to DNA just
anywhere.
• The enzyme only binds to a region called
a promoter, which has a specific sequence
or “start codon.”
– Promoters: Signals in DNA that indicate to
the enzyme where to bind to make RNA.
• Similar signals in DNA cause transcription to stop.
RNA Editing
• When RNA is made from DNA, there is a lot of
editing that needs to be done.
– RNA Polymerase incorrectly copies 1:10,000
nucleotides.
• DNA contains sequences of nucleotide that do
not code for proteins. (Introns)
• The DNA sequences that do code for proteins
are called Exons.
– These are expressed in protein synthesis.
– When the RNA is being created from DNA, both
introns and exons are copied.
– Introns must be cut out and then the exons are
spliced together.
– The information that is left from the exons can
then be translated into a protein.
The Genetic Code
• The properties/characteristics of proteins are
determined by the order in which amino acids
are joined together to make a polypeptide
– Polypeptide= A long chain of amino acids.
• The genetic code is read 3 letters at a time
– Each 3-letter word is called a codon
– Codon: consists of 3 consecutive nucleotides that
specify a single amino acid.
• 64 possible codons.
• 1 start codon
• 3 stop codons
• What is the start
codon? Stop codons?
Crack the code:
•
UCGCACGGU
Translation
• Translation: Process of decoding an mRNA message
into a protein
– Takes place on ribosomes
Step 1: messenger RNA
• After transcription, mRNA leaves the nucleus
and enters the cytoplasm.
Step 2:
• mRNA attaches to a ribosome.
• mRNA moves through the ribosome’s AP site,
the codon sequence is read, and tRNA brings in
the matching amino acid, which is added to the
polypeptide.
tRNA
• Each tRNA molecule carries only 1 kind of amino
acid
• Each tRNA has 3 unpaired bases, which are
complimentary to an mRNA codon= anticodon
Step 3
• Peptide bonds are formed between the amino acids
in the AP site.
• The ribosome moves on to the next codon, the first
tRNA exits the ribosome, and the bond between the
amino acid and the tRNA is broken.
• A new tRNA enters the empty A site.
Step 4
• Polypeptide chain grows until a stop codon is
reached.
• When this happens, translation ends and the
polypeptide folds into a new, useable protein.
Summary
• DNA contains a code for the creation of
proteins.
– This happens through a series of steps.
• DNA needs to be transcribed into a readable
code
– Transcribe: To write out in another language.
• The newly transcribed code then needs to be
translated, or interpreted, so a protein can be
made.
Protein Synthesis Overview
• Crash Course
• The Hippies
Genetic Mutations
Genetic Mutations
Intro.
• Cells sometimes make mistakes when they are
copying their own DNA.
– Mutation: An accidental change in the genetic material.
– 2 main categories for mutations:
• Gene Mutations: Produce changes in a single gene.
• Chromosomal Mutations: Produce changes in whole
chromosomes.
Gene Mutations
• Point Mutation: Involves changes in one or a few
nucleotides.
– Occur at a single point in the DNA sequence.
– Types: Substitutions, Insertions, and deletions.
• Substitutions are not that bad for the protein.
– Ex: ATT CGT ACA GAT  ATT CGT ACC GAT
• Insertions and deletions can ruin the protein by
causing the entire sequence to shift, changing all of
the amino acids.
– Frameshift Mutation
– Ex: ATT CGT ACT GAT  ATT CTA CTG AT (Deletion)
 ATT CGG TAC TGA T (Insertion)
Chromosomal Mutations
Significance of Mutations
• Most mutations have no real affect on an organism.
• When protein structure is altered, most of the time it is
harmful
– Cancer, fatality
• Sometimes these are beneficial and lead to genetic
variation and evolution.