DNA: (Deoxyribonucleic

Acid)The Genetic material

of all living organisms

Inheritance has its molecular basis in the precise replication and transmission of DNA from parent to offspring.

Was first discovered by Frederick Meissner (german physician) in 1860s while he was changing puss covered

wound dressings

1928:Frederick Griffith

• Performed experiments which provided evidence that genetic material is a particular chemical

• Griffith was trying to find a vaccine against Streptococcus pneumoniae( bacteria that causes pneumonia)

Griffiths Experiment

• Experiment: inject live S strain into mice

• Results: mice die

• Conclusion: strain is pathogenic

Griffiths Experiment

• Experiment: inject mice with live R strain

• Results: mice live

• Conclusion: R strain not pathogenic

Griffiths Experiment

• Experiment: Inject mice with heat-killed S strain

• Results: mice live

• Conclusion: heat kills bacteria and it is no longer pathogenic

Griffiths Experiment

• Experiment: mix heat killed S strain with live R strain

• Results: mice die

• Conclusion: something chemical in nature moved from the S strain to the R strain making it pathogenic

History

•By the 1940s, scientist know that chromosomes carried hereditary material that consisted of DNA and protein.

•Most researchers thought that protein was the genetic material

• 1944 Demonstrated that DNA alone from virulent bacterium can transform nonvirulent bacterium.

• Provided more evidence that DNA is genetic material came from studies of bacteriophages ( virus that infects bacteria)

• 1947 Developed Chargaffs rule of pairing nucleotides:

• Adenine---Thymine

• Cytocine----Guanine

Watson and Crick built a scale model of a double helix that would conform to data and known chemistry of DNA

They proposed a ladder-like molecule that twisted into a spiral ( double helix), with sugar and phosphate backbones and nitrogen base pairs as rungs of the ladder

In April 1953, Watson and Crick’s new model of DNA structure was published in the British Journal of Nature.

This model for the DNA structure suggested a template mechanism for DNA replication…and was called a semiconservative model for DNA replication

semiconservative model for DNA replication

• Watsons and Crick’s model suggested:

1. The 2 DNA strands separate

2. Each strand is a template for assembling a complementary strand

3. Nucleotides line up along the template according to base pairing rules

4. Enzymes link the nucleotide together

5. Each new DNA strand has one old and one new strand

• Rosalind Franklin's x-ray diffraction work gave Watson and Crick the key to unlocking the structure of DNA.

• She died before she could receive appropriate recognition of her contribution. She did not receive the Nobel Prize (not administered posthumously).

Martha Chase & Alfred Hershey (1952)

Martha Chase & Alfred Hershey (1953)

Find that only DNA from virus enters cells and

directs reproduction of new viruses

The Meselson - Stahl Experiment 1958

"DNA Replication is Semiconservative

• They proposed that when the time came for DNA to be replicated, the two strands of the molecule:

1. separated from each other but 2. remained intact as each served as the

template for the synthesis of 3. a complementary strand.• When the replication process is

complete, two DNA molecules — identical to each other and identical to the original — have been produced

History• DNA

– Comprised of genes– In non-dividing cell

nucleus as chromatin• Protein/DNA complex

– Chromosomes form during cell division

• Duplicate to yield a full set in daughter cell

History• DNA• The 5 carbon sugar

(PENTOSE) is Deoxyribose

• It is double stranded• DNA is found in the

nucleus of the cell• Composed of sugar,

phosphate and nitrogen base pairs

DNA is a Double Helix

• Nucleotides– A, G, T, C

• Sugar and phosphate form the backbone

• Bases lie between the backbone

• Held together by H-bonds between the bases– A-T – 2 H bonds– G-C – 3 H bonds

H - Bonds • Base-pairing rules– AT only (AU if DNA-RNA

hybrid)– GC only

• DNA strand has directionality – one end is different from the other end

• 2 strands are anti-parallel, run in opposite directions– Complementarity results– Important to replication

Helical Structure

Nucleotides as Language

• We must start to think of the nucleotides – A, G, C and T as part of a special language – the language of genes that we will see translated to the language of amino acids in proteins

Genes as Information Transfer

• A gene is the sequence of nucleotides within a portion of DNA that codes for a peptide or a functional RNA

• Sum of all genes = genome

• DNA collected from cheek cells

General mechanism of DNA replication is conceptually

simple, but the actual process is complex.

DNA Replication

• Semiconservative

• Daughter DNA is a double helix with 1 parent strand and 1 new strand

• Found that 1 strand serves as the template for new strand

DNA Template

• Each strand of the parent DNA is used as a template to make the new daughter strand

• DNA replication makes 2 new complete double helices each with 1 old and 1 new strand

• Step 1: Origin of replication• Step 2: Strand separation• Step 3: Priming• Step 4: Synthesis of new DNA Strand• Step 5: Proof reading

Steps 3 and 4 happen simultaneously

DNA replication begins at a special site call the origin of replication

The ‘DNA double helix open at the origin and replication forks spread in both directions away from the central initiation point creating a replication bubble.

Replication Origin

• Site where replication begins – 1 in E. coli– 1,000s in human

• Strands are separated to allow replication machinery contact with the DNA– Many A-T base pairs

because easier to break 2 H-bonds that 3 H-bonds

• Note anti-parallel chains

When DNA replicates, many different proteins work together to accomplish the following steps:  

The two parent strands are unwound with the help of DNA helicases ( an enzyme).

• Single stranded DNA binding proteins attach to the unwound strands, preventing them from winding back together.

• Topoisomerase: Is another protein that serves to relieve the stress and untangle any kinks or snarls in the DNA

• The strands are held in position, binding easily to DNA polymerase, which catalyzes the elongation of the leading and lagging strands. (DNA polymerase also checks the accuracy of its own work!).

• All is according to the base-pairing rule

• While the DNA polymerase on the leading strand can operate in a continuous fashion, RNA primer is needed repeatedly on the lagging strand to facilitate synthesis of Okazaki fragments.

•

• DNA primase, which is one of several polypeptides bound together in a group called primosomes, helps to build the primer.

• A primer is a short segment of RNA made by an enzyme called primase

• Finally, each new Okazaki fragment is attached to the completed portion of the lagging strand in a reaction catalyzed by DNA ligase.

• There are more than 50 different types of DNA repair enzymes, but most perform 1 of 2 different types of repair.

• 1. Directly reversing the change.

• 2. Excision repair: The damage segment is excised by one repair enzyme and the remaining gap is filled in by base-pairing nucleotides

• Legend:

DNA and RNA transcription video - real time DNA encoding pr

• http://DNA and RNA transcription video - real time DNA encoding pr - YouTube

Transcription and translation are the means by which cells read out, or express, the genetic instructions in their genes.

Because many identical RNA copies can be made from the same gene, and each RNA molecule can direct the synthesis of many identical protein molecules, cells can synthesize a large amount of protein rapidly when necessary

Transcription begins with the opening and unwinding of a small portion of the DNA double helix to expose the bases on each DNA strand.

One of the two strands of the DNA double helix then acts as a template for the synthesis of an RNA molecule.

As in DNA replication, the nucleotide sequence of the RNA chain is determined by the complementary base-pairing between incoming nucleotides and the DNA template.

• Unlike a newly formed DNA strand, the RNA strand does not remain hydrogen-bonded to the DNA template strand. Instead, just behind the region where the ribonucleotides are being added, the RNA chain is displaced and the DNA helix re-forms.

• Thus, the RNA molecules produced by transcription are released from the DNA template as single strands.

• The enzymes that perform transcription are called RNA polymerases

• Transcription begins when the enzyme RNA polymerase binds to the promoter,( a specific nucleotide sequence ) located in the DNA at the start end of the gene and continues until a terminator code is reached.

• Translation is a process where genetic information is translated from a ``nucleic acid language" to an "amino acid language". 

• Translation is catalyzed by a large enzyme called a ribosome, which contains proteins and ribosomal RNA (rRNA). 

• Translation also involves specific RNA molecules called transfer RNA (t-RNA) which can bind to three basepair codons on a messenger RNA (mRNA) and also carry the appropriate amino acid encoded by the anticodon ( tRNA).

•   The ribosome assembles on the first AUG (start codon) in the mRNA.  This codon encodes the amino acid methionine (Met).