Chapter 10

Nucleic Acids

& Protein

Synthesis

•After completing the chapter on Genetics, we discussed the passing on of genes, but how are genes produced?

Brief history of DNABrief history of DNA Frederick GriffithFrederick Griffith – 1928 • Experimented with pneumonia bacteria calledStreptococcus pneumoniae. There were two strains of

the bacteria:

• 1 formed smooth colonies and caused pneumonia• The other formed rough colonies and was harmless. Heat kills disease causing strain - mice don’t get pneumonia. Adds heat killed strain to the harmless strain injected into mice. The

mice die.

He cultures the bacteria. Finds that the harmless rough bacteria had been “transformed” or changed into the lethal strain.

Griffith’s Transformation Experiment

Conclusion

• The harmless bacteria were transformed by some factor from the harmful bacteria = this is called transformation

• Did not know what that factor was though

Oswald Avery 1944

• Repeated Griffith’s work but used enzymes to destroy proteins in the heat killed bacteria.

• Pneumonia and transformation still occurred. • Then used an enzyme to destroy RNA. Still

transformed occurred. • Finally, used an enzyme to break apart DNA. • This stopped transformation!!!

Conclusion

• DNA is the nucleic acid that stores and transmits genetic information.

Hershey and Chase – 1952• Used T4 bacteriophage virus that infects E. coli

bacteria• A virus is a non-living pathogenic particle that can’t

replicate on its own

Capsid (protein)

The Lytic Cycle of Virus infection

Attaches onto host cell Injects DNA into host cell Replication of Viral parts

Reassembly of virons Lysis – bursting out

What part of a virus actually infects & causes the host cell to become a viral

factory?

Used radioactive isotopes of phosphorus and sulfur

P32 and S35

Proteins do not have Phosphorus DNA doesn’t contain Sulfur

Used S35

Used P32

Found that the S35 stayed outside the cell & P32 ended up in new virons

Animation

Conclusion

The genetic material of the bacteriophage is located in the DNA,

not the protein coat

Structure of DNAStructure of DNADeoxyribonucleic acid

Polymer of the monomer – Nucleotides

Single nucleotide

5 carbon sugar –deoxyribose

A phosphate group

A nitrogen baseS

P

N-base

Nucleotide Sugar & phosphate alternate to make up the sides of the strand

Found only in nucleus

Erwin Chargaff – 1940’s

Noticed a pattern in the amounts of the four bases: Adenine, Guanine, Cytosine, and Thymine

• Found the number of Guanine & Cytosine nitrogen bases is always equal in DNA

• & the number of Thymine and Adenine is always equal. • Didn’t know why though!

4 nitrogen bases •Guanine - Purine

•Cytosine - Pyrimidine

•Adenine - Purine

•Thymine - Pyrimidine

Follow base pairing rule Adenine with Thymine Guanine with Cytosine

Bases are held together by weak hydrogen bonds

N-bases connect to sugars by a covalent bond

5 Carbon sugar

Phosphate group

Nitrogen base

Weak H bond

Covalent bond

History of DNAHistory of DNA:

• Rosalind FranklinRosalind Franklin took X-Ray diffraction photo of DNA.

Watson and CrickWatson and Crick (1953) • Using Franklin’s photo, came up with the

double helix form of DNA. Won Nobel Price w/ Maurice Wilkins (1962).

Original DNA model.

So, how does DNA replicate?

• Occurs during S phase of interphase – DNA makes two exact copies of the original; if not, a mutation occurs.

STEPS:

1. The double helix unwinds and flattens out (like a zipper)2. An enzyme called DNA helicase (like the zipper slide) unzips the strand at the weak

hydrogen bonds. This exposes the nitrogen bases (each tooth of the zipper).3. Another enzyme called DNA polymerase will be responsible for rezipping the strands.

It will take free nucleotides in the nucleus and bond them to the exposed bases, following the base pair ruling – G – C and A – T. EACH SIDE OF THE MOLECULE ACTS AS A TEMPLATE FOR A NEW STRAND.

4. The base pairing continues until the entire strand has their complement.5. Now there are two identical strands of DNAhttp://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter3/

animation__dna_replication__quiz_1_.html

SUMMARY:

* DNA helicase unzips the double strand

* Original (old) strands of DNA are on the outside of the new strands – THESE ARE THE TEMPLATES

* Replicates takes place in opposite directions with the help of DNA polymerase

* Semi-conservative model

How good is DNA at replicating ?

• Accurate to about 1 error for every 1,000,000,000 base pairs.

• Why? Two reasons: complementarity and DNA polymerase, the “proofreader!”

• Gene Mutation – error resulting from misreading of DNA or problem in the transcription/translation process (We’ll revisit this later)

• Mutations in genes that control cell division or repair enzymes may cause cancer

The Flow of Genetic Information

• DNA cannot leave the nucleus – it uses a helper molecule called RNA

• DNA is the template for RNA = transcription• RNA then directs the synthesis of proteins on ribosomes =

translation• DNA RNA protein

RNA Ribonucleic acidRibonucleic acidThe other Nucleic Acid

•Acts as a messenger between DNA and the ribosomes and carries out protein synthesis

•DNA is too large to get out of the nucleus; it is also protected in the nucleus from DNases. The cell uses RNA to bring its message to the rest of the cell for protein synthesis

How DNA & RNA Differ:

* RNA is a single stranded molecule

*RNA has ribose sugar instead of deoxyribose

*RNA contains Uracil in place of Thymine so Adenine bonds with Uracil

*RNA can be found in the nucleus, cytoplasm

or at the ribosomes

Let’s Review!!!

• Ribosomes are small organelles that are involved with making proteins

• They are made up of proteins and rRNA• They consist of two subunits – large and small• Ribosomes are found both in the cytoplasm and on the

endoplasmic reticulum

There are three different kinds of RNA

• Messenger RNA (mRNA)Formed in the nucleus and goes to the

ribosomes; carries genetic code from DNA through the cytoplasm to the ribosomes

• Transfer RNA (tRNA) Shaped like T; carries amino acids to the mRNA

on the ribosomes • Ribosomal RNA (rRNA)

Most abundant; found in globular form (like a big glob) and makes up the ribosomes

The Process of Protein The Process of Protein SynthesisSynthesis* Process by which DNA codes for the production of proteins

(polypeptide chains) and protein assembly- Polypeptide chains are polymers of the 20 different amino acids.

- Uses a genetic code – chemical letters in RNA that make up words which code for particular amino acids

- Check your understanding: what happens if the letters change?

Transcription

Translation

Polypeptide formingAmino Acid

Part I. Transcription of DNA into mRNA (the message)• DNA flattens and is unzipped exposing its bases (template) – sound

familiar? • RNA polymerase binds free RNA nucleotides to exposed DNA bases

starting at a promoter – a specific DNA nucleotide pattern• Complementary base pairing occurs, EXCEPT THERE IS NO THYMINE IN

RNA. Instead, Adenine bonds with Uracil just as Thymine from DNA would bond with Adenine.

• Transcription continues until a termination signal is given (punctuation) to stop the transcription process

• If DNA reads: ATC GTC GAT TGG C AA• mRNA: UAG CAG CUA ACC GUU• mRNA leaves the nucleus through a pore to go out into the

cytosol to locate a ribosome• FYI – any of the three types of RNA are made this way• http://www-class.unl.edu/biochem/gp2/m_biology/animation/

gene/gene_a2.html

The Genetic Code:• Where a group of 3 nucleotide bases translates into a particular

amino acid• This 3 “letter word” is called a codon• Codons are groups of 3 adjacent bases on mRNA (AAA, CCC

GGG)• Each codon will specify a specific amino acid. • When the codon is recognized by the anticodon, this is called

Translation• There are 64 different codons with punctuation as well for start

and stop

Start codon

Stop Codons

About the genetic code…

•Codons are code words found in mRNA•Codons code for particular amino acids•Three of the 64 codons are stop, one is start – AUG = methionine

•The code is degenerate – more than one codon can code for an amino acid – why is this important?

•The code is UNIVERSAL!!!

The questions are: what is an anticodon and how does the amino acid get selected?

Part II. Translation of mRNA into protein* At the ribosome, the process of translation occurs.

Several ribosomes may undergo this process at one time

• mRNA will temporarily bind with the two ribosomal subunits

• tRNA is waiting in the cytoplasm with its corresponding amino acid

• Starting with the start codon (AUG), in groups of 3, mRNA will determine which amino acid tRNA must bring to the ribosome.

• Animation – Virtual Cell

Transcription

Translation

Polypeptide forming

• Once tRNA brings the correct amino acid to mRNA at the ribosome, it releases and goes back to the cytoplasm to pick up it corresponding amino acid

• Adjacent amino acids bond together, making a peptide bond to form a polypeptide.

• Chain could be up to 10,000 amino acids long

• This continues until the entire message is translated.

• The chain of amino acids is formed called a polypeptide (protein). The translation ends when a STOP codon is reached (UAA, UAG, UGA).

When things go wrong:

• Does this process ever make a mistake?• Have you ever had to copy a large amount of

information? • What is the likelihood of you making a mistake or more?• What could cause these changes?

Changes in genetic material

Gene Mutations: alters one or more genes

Chromosomal Mutations:alter the entire chromosome or a

portion of it.

Gene Mutations

Point MutationsPoint Mutations – affect only one amino acid

Frameshift mutationsFrameshift mutations – May affect an entire amino acid sequence.

Point mutation

• involves a change in one or a few nucleotides. • Influences a single amino acid in the polypeptide change; caused by

a substitution of a nitrogen base. • Sickle cell anemia is an example of this – GUG instead of GAGValine instead of glutamic acid

• THE FAT CAT ATE THE RAT • Take out “C” in Cat & substitute a “B” • THE FAT BAT ATE THE RAT• In this case, it does not really change the meaning to the sentence

or the protein formed

• If DNA reads: A T G G T C G A T T G G CAA• mRNA: U A C C A G C U A AC C GUU• Amino Acid: Tyrosine - Glutamine – Leucine -Threonine – Valine

• But if mRNA: U A C C A G C A A AC C GUU• The AA: Tyrosine – Glutamine – Glutamine – Threonine – Valine

Frameshift mutation

•involves a change in the entire protein formed or a large portion of it.

•Caused by insertions (additions) or deletions of nitrogen bases.

•Tay-Sachs is a disease caused by a frameshift mutation

•THE FAT CAT ATE THE RAT •Take out “E” in THE & group into 3’s •THF ATC ATA TET HER AT_This makes no sense at all!!

• If DNA reads: A T G G T C G A T T G G CAA• mRNA: U A C C A G C U A AC C GUU• AA: Tyrosine - Glutamine – Leucine -Threonine – Valine

• BUT if mRNA: U A C C A G U A A C C G U U _• THEN Amino Acid: Tyrosine - Glutamine – STOP!!!!• The entire sentence makes no sense. The protein formed would be

totally different

So which form of a mutation would be more severe?

• Frameshift mutationFrameshift mutation … …since an entirely new protein would be formed

CHROMOSOMAL MUTATIONS

•involve changes in number and structure of the chromosomes.

•Could change location of genes on the chromosomes or the number of copies of some of the genes.

• Deletions – part of a chromosome is missing

Duplications – Extra copies of genes are inserted

• Inversions – Reverse direction of parts of the chromosome

Chromosomal Mutations animation

Parts of one non-homologous chromosome breaks off and attached onto another non-homologous chromosome

Translocations