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.