Chapter 8From 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 ReplicationThe 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 ReplicationThe 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 ReplicationThe 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 semi-conservative 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 proteins1) 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.