Download Chapter 8

Chapter 8
From DNA to Proteins
Section 8.2: Structure of DNA
• Since the 1920’s scientists have known that DNA
is a very long polymer (chain of repeating units).
• The small units that make up DNA are called
nucleotides.
• Remember, nucleotides are made up of 3 parts:
– A phosphate group
– 5-carbon sugar called deoxyribose
– A nitrogen base (Guanine, Cytosine, Thymine, or
Adenine)
Section 8.2: Structure of DNA
• To give you an idea of the size, one molecule
of DNA contains about a billion nucleotides.
• For a long time scientists believed that
organisms were made up of equal amounts of
four different types of nucleotides.
– For example: humans were 25% Guanine, 25%
Thymine, 25% Adenine, and 25% Cytosine.
Section 8.2: Structure of DNA
• By 1950, Erwin Chargaff changed the
thinking on DNA. Chargaff studied
several different organisms and found
that the same 4 bases are in all
organisms, but the proportion of the
bases varied.
• He found that in all organisms that
– the amount of adenine=the amount of
thymine
– that the amount of guanine=the
amount of cytosine.
• A=T & C=G became known as
Chargaff’s Rule
Section 8.2: Structure of DNA
• Thymine and cytosine are single ring
structures called pyrimidines
• Adenine and guanine are double ring
structures called purines
Section 8.2: Structure of DNA
• Remember, it can’t just be a pyrimidine
bonding with a purine…..it is more specific
than that…. (A) always bonds with (T) and (G)
always bonds with (C).
Section 8.2: Structure of DNA
• Finally, in the early 1950’s, a
complete understanding of
DNA was finally coming into
focus.
• Rosalind Franklin was studying
DNA at the time.
• Franklin took x-ray
photographs of DNA and it
showed it to be in an “X”
form.
Section 8.2: Structure of DNA
• At the same time James
Watson and Francis Crick
were studying DNA and saw
Franklins work.
• They both cam to the
conclusion that the picture,
if put in 3-dimension, the
“X” form would be twisted
on itself like a spiral
staircase (helix).
Section 8.2: Structure of DNA
• Watson and Crick found the sugar and
phosphates were the outside backbone of the
molecule and the nitrogen bases were on the
inside.
• In 1953, Watson and Crick published their
DNA double helix model.
• It shows a two-stranded molecule wrapped
around each other held together by hydrogen
bonds between adjacent bases.
Section 8.2: Structure of DNA
• The model shows the two strands interwinded
with each other.
• It also shows the complimentary bases paired.
• The back ribbon-like part is the phosphates
and 5 carbon sugar deoxyribose.
– Because of their unique structures, adenine can
only bond with thymine and cytosine with
guanine!
Section 8.3: DNA Replication
• Watson and Crick’s model was
also important because it
suggested a way DNA could be
replicated.
• Both scientists suggested that
because of the base pairing rules
(A-T & C-G), each strand could
serve as a template to make a
copy of the other strand.
• This process is called DNA
replication.
Section 8.3: DNA Replication
• DNA replication insures that every cell has a
complete set of identical genetic information.
• Enzymes and other proteins do the actual
work of DNA replication.
• A group of enzymes called DNA polymerases
guide this 3 step process.
Section 8.3: DNA Replication
The process of DNA replication can be described in
3 steps:
1) Enzymes begin to “unzip” the double helix. This
means the hydrogen bonds between the
nitrogen bases are broken. When these
hydrogen bonds are broken, the two strands
separate and each individual base is exposed.
Like unzipping a suitcase, it proceeds in two
directions at the same time.
Section 8.3: DNA Replication
The process of DNA replication can be described in
3 steps:
2) One by one, free floating nucleotides pair with
their exposed complimentary case. DNA
Polymerase bond the nucleotides together to
make a new strand
* Each strand is a template to make the other
strand.
Section 8.3: DNA Replication
The process of DNA replication can be described in
3 steps:
3) Two identical molecules of DNA are the end
result. Each molecule is made up of one new
strand and one old strand. This is called semiconservative replication.
*Because of semi-conservative replication,
something amazing happens! What is it??
Section 8.3: DNA Replication
• DNA replication happens over and over again in
every cell in your body.
• This process also happens remarkably fast,
about 50 nucleotides per second.
• The only way it gets done is that replication
occurs at multiple places on DNA at one time!
Section 8.3: DNA Replication
• There is also a built in proofreading system.
• This system corrects any mis-paired nucleotides.
• The error rate is about 1 out of 1,000,000,000
because of the proofreading!
Section 8.4: Transcription of DNA
• Francis Crick defined the Central Dogma of biology
after the discovery of the structure of DNA.
• This states that information flows in one direction,
from DNA  RNA  Proteins.
• The central dogma involves 3 processes:
– Replication: of DNA strands
– Transcription: converts DNA messages into RNA
language
– Translation: interprets RNA language into a string of
amino acids called polypeptides. These polypeptides
working together make up proteins.
Section 8.4: Transcription of DNA
• In prokaryotic cells (bacteria), all 3 processes
occur in the cytoplasm at the same time.
• In eukaryotic cells, replication and transcription
occur in the nucleus and translation occurs in
the cytoplasm at different times.
• RNA or Ribonucleic Acid acts as a link between
the DNA in the nucleus and protein synthesis in
the cytoplasm.
Section 8.4: Transcription of DNA
• RNA is similar to DNA in that it is a chain of
nucleotides made up of sugar, phosphates, and
nitrogen base.
• You can think of RNA as a temporary copy of
DNA that is used and then destroyed.
• RNA, while similar to DNA, differs in 3 significant
ways:
– The sugar in RNA is ribose sugar which has oxygen
– RNA contains the base Uracil instead of Thymine
– RNA is only a single strand
Section 8.4: Transcription of DNA
• By definition, transcription is the process of
copying a sequence of DNA to produce a
complimentary strand of RNA.
– RNA strands only copy the segment of DNA it needs
to make a specific gene.
• During transcription, the whole DNA code is not
copied. Only the code for the specific gene
needed is copied.
Section 8.4: Transcription of DNA
• The process is helped along by RNA
polymerases, which are enzymes that bond
nucleotides to make an RNA strand.
• There are 3 basic steps to transcription:
Section 8.4: Transcription of DNA
• There are 3 basic steps to transcription:
1) RNA polymerase recognizes the transcription
start site for a specific gene. A large transcription
complex (RNA polymerase and other proteins)
assembles on the DNA strand and begins to
unwind the DNA segment needed.
Section 8.4: Transcription of DNA
• There are 3 basic steps to transcription:
2) RNA polymerase, using only one strand of DNA,
strings together complimentary strand of RNA
nucleotides. RNA follows the same base pairing
rules as DNA, however, RNA contains the base
uracil, not thymine. As the RNA strand is made,
the DNA helix zips back up behind it.
Section 8.4: Transcription of DNA
• There are 3 basic steps to transcription:
3) Once the entire gene has been transcribed, the
RNA strand detaches completely from the DNA.
RNA polymerase recognizes the end of the gene
and transcription is stopped.
Section 8.4: Transcription of DNA
• Transcription produces 3 major types of
RNA…not all RNA molecules code for proteins
1) Messenger RNA (m-RNA)- the molecule that
carries the transcribed message from DNA to
the ribosomes to make proteins.
2) Ribosomal RNA (r-RNA)- forms part of the
ribosomes
3) Transfer RNA (t-RNA)- brings amino acids from
the cytoplasm to the ribosomes to put the
protein together.
Section 8.4: Transcription of DNA
• The transcription process is similar to replication
of DNA.
• Both occur in the nucleus, catalyzed by enzymes,
unwind DNA, and produce complimentary base
pairs.
• However, the end results of replication and
transcription is very different.
Section 8.5: Translation
• Translation is the process that converts a mRNA
message into a protein.
• The language of nucleic acids is A,C,T,G, & U’s,
but the language of proteins is amino acids
(remember these are the monomers of proteins)
• The A,C,T,G, &U’s of a nucleic acid are in a very
specific order.
• Every 3 letters is a triplet, or Codon.
Section 8.5: Translation
• A codon is a 3 nucleotide sequence that codes
for a specific amino acid.
• Scientist believe it is every 3 nucleotides because
that gives enough possible nucleotide
combinations to cover all 20 amino acids.
• Amino acids are generally coded for by more
than one possible codon.
– For example: CUU, CUA, CUG, UUA, & UUG are all
codes for Leucine, one of the 20 amino acids
Section 8.5: Translation
• So codons are every 3 nucleotides, so every 3
nucleotide makes another amino acid.
Section 8.5: Translation
• There are two other types of codons other than
the ones that code for proteins.
• There are 3 stop codons (UUA, UAG, & UGA).
• These codons signal the stopping of an amino
acid sequence.
• There is also one start codon (AUG) that triggers
the start of translation.
• AUG also codes for an amino acid (Methionine),
so translation always starts with this amino acid.
Section 8.5: Translation
• If, during translation, a codon is read wrong or
one nucleotide is incorrect, this could affect the
whole protein!
• The genetic code is shared by almost all
organisms.
• For example: UUU codes for Phenylalanine in
humans, a cactus, yeast, or an armadillo.
Section 8.5: Translation
• This makes most scientist believe that all living
organisms gave rise from a common ancestor.
• It also means scientists can use a gene from one
organism in different organisms.
• So , how do we get a protein made from the
instructions mRNA carries to the ribosomes?
• This process is called translation.
• Translation actually has many steps, requires a
lot of energy, and is a complicated process.
Section 8.5: Translation
• We will try to summarize translation in 3 steps:
1) The codons on the mRNA reach the ribosomes
and attracts a complementary tRNA molecule
that carries an amino acid. The tRNA anticodon
pairs with the mRNA codon.
Section 8.5: Translation
• We will try to summarize translation in 3 steps:
2) The ribosome helps form bonds between amino
acids. It then breaks the bond between the tRNA
and amino acid.
Section 8.5: Translation
• We will try to summarize translation in 3 steps:
3) The ribosome continues to pull the mRNA
strand through the ribosome until all the codons
are read and matched with their tRNA anticodon
and amino acids. The amino acids are then all
connected to make a protein.
Section 8.5: Translation
• Below is an example of transcription and
translation working together to make a protein.
Section 8.7: Mutations
• So, what happens when something goes wrong?
– A mutation occurs
• A mutation is a change in an organisms DNA.
• There is many different types of mutations.
• Mutations usually happen during DNA
replication (usually affects a single gene)
• Or during meiosis (affects entire chromosomes)
Section 8.7: Mutations
• Gene Mutations:
– Point Mutations: this type of mutation happens
when one nucleotide is substituted for another
• ACTG is copied as TGAA (last nucleotide is mispaired)
– Frameshift Mutation: involves a deletion or insertion
Section 8.7: Mutations
• Chromosomal Mutations:
– These type of mutations affect parts or a whole
chromosome.
– Deletion: Part of a chromosome is missing
– Duplication: part of a chromosome is copied twice
– Inversion: part of a chromosome switches places
– Translocation: pieces of one chromosome moves to
a non-homologous chromosome.
Section 8.7: Mutations
• Although there are many types of mutations, the
affect isn’t always bad!
• Silent mutations are changes in an organisms
DNA that do not change anything.
• Also, only mutations that happen in gametes
(sperm & egg) affect an organisms offspring.
• If a mutation occurs in a body cell, that mutation
only affects that organism.
Section 8.7: Mutations
• Mutagens are agents in the environment that
cause mutations.
• They speed up replication errors and your body’s
proofreading system.
– Examples: UV light, chemicals in cigarettes, and
other chemicals.