Molecular Biology

DNA Structure

DNA Replication

RNA Structure

Transcription

Translation

The DNA Puzzle Scientists knew chromosomes were composed of

DNA and proteins They did not know which one actually stored

genetic information Most assumed proteins because their structure was

so much more varied and complex In 1928 Griffith performed an elegant experiment

which proved bacterial transformation could occur 1944 Avery Mcleod and McCarty proved the

agent that transformed the bacteria was DNA

Chargaff in 1947 determined that nitrogen base percentages were species-specific and that A=T and C=G was true for DNA

In 1952, Hershey and Chase used bacteriophages to show that DNA was genetic material using radioactive tags for sulfur and phosphorus

Wilkins and Franklin determined by x-ray crystallography photographs that width of the molecule was 2nm with 2 strands

Watson and Crick built models to conform to experimental results and came up with a double helix in which pyrimidines and purines were paired.

DNA Structure You should already know or recall that DNA is

composed of nucleotides, each having one of four nitrogen bases: adenine, thymine, cytosine, and guanine.

The shapes of the nitrogen bases fit together in such a way that they are complementary. Adenine always bonds to Thymine and Cytosine always bonds to Guanine.

Sugar-phosphate bonds form the backbone of the helix and hydrogen bonds form the bonds between the nitrogen bases.

DNA

When DNA must replicate, each side is a template for a new strand. This is the semiconservative model of DNA replication

It means that one side of each of the two original strands builds a complementary strand so that two identical DNA molecules result.

This is possible because of the base-pairing rules (A always bonds to T and C always bonds to G)

DNA Replication There are a number of things associated with the

replication of DNA that you must remember-think of them as rules:

1. A new DNA strand can only elongate in the 5`-----3` direction.

2. Each original strand serves as a template for the formation of a new strand. (Semiconservative Model)

3. The leading strand elongates continuously while the lagging strand forms in pieces (called Okazaki fragments) which are joined by DNA ligase.(another enzyme)

DNA Replication (cont.)

4. The replication bubble forms when proteins attach to a specific sequence of nucleotides to begin the process.

5. Along with the replication bubble, a replication fork is present and is where the DNA strands are elongating.

6. DNA Polymerases (enzymes) catalyze the elongation of the DNA strands.

7. Nucleoside triphosphates supply the energy for the process.

8. DNA synthesis is primed by RNA and an enzyme called primase.

Repair and Keeping Ends Intact

DNA repair is accomplished by using nucleases (enzymes) to cut out a damaged part of DNA and then filling it in with the correct sequence.

The ends of the DNA molecule have telomeres, strands of non-coding DNA that keep the ends from becoming shorter as the DNA replicates again and again. An RNA primer sequence and the enzyme telomerase accomplish this task.

RNA Structure As you should recall, there are a number of

differences between DNA and RNA. The first is that the sugar in RNA is ribose, not

deoxyribose. The second is that instead of thymine, RNA has

the base uracil, which bonds to adenine. Third, it has 3 different shapes related to each

form of RNA. (DNA has only one shape) Messenger RNA is linear, Transfer RNA is shaped

like a cloverleaf, and Ribosomal RNA is globular.

RNA’s Functions

mRNA carries the genetic information from DNA into the cytoplasm of the cell. Remember that DNA does not leave the nucleus.

tRNA delivers amino acids to the ribosome to be added to the forming polypeptide chain.

rRNA makes up ribosomes where the polypeptide chain is assembled.

Transcription-Making mRNA from DNA Transcription has three steps: Initiation Elongation Termination In initiation, RNA polymerase, an enzyme,

attaches to special regions on the DNA known as Promoter Regions which are also known as the TATA Box because those are the nucleotides found there. At this site, the DNA is unzipped into two strands.

Transcription (cont.)

Elongation occurs when the RNA polymerase assembles RNA nucleotides using one side of the DNA as a template.Only one side of the DNA molecule is used and the lengthening strand moves in the 5` to 3` direction as DNA replication does.

Termination occurs when the RNA polymerase reaches a sequence of DNA (often AAAAAAA) called the Terminator region.

Modifications to mRNA

Before it leaves the nucleus, the mRNA is altered. GTP is added to the 5` end forming a “cap”. 150-200 adenine nucleotides are added to the 3`

end to form a “poly A tail”. (The two above acts are believed to be related to gene expression)

Introns(sequences between coding sequences called exons) are removed.

Small ribonucleoproteins (snRNP’s) are the “cutters” that remove the noncoding sequences.

Translation-from mRNA to Polypeptide All the RNA players move to the

cytoplasm. (mRNA, tRNA, ribosome subunits)

An enzyme specific to each amino acid and ATP attach the correct amino acid to the 3` end of tRNA.

The remaining sequence of events follows on the next several slides.

Initiation of Translation The small ribosomal subunit attaches to a special

region near the 3` end of the messenger RNA molecule.

A tRNA with the nucleotide sequence UAC on its anticodon carries the amino acid

methionine which attaches to the “start” codon AUG on the mRNA.

The large subunit of the ribosome attaches to the small, and the mRNA forming a complete ribosome with the tRNA at the “P” site.

Elongation of Translation The next tRNA comes with an amino acid which binds to the A site on

the ribosome. The release of the methionine and its addition to the next amino acid

occurs, beginning the chain of amino acids that will eventually form the polypeptide.

As each tRNA is released, the former one moves to the P site, exposing the A site which allows a new codon to be brought to it. So a new tRNA with a new amino acid enters the A site.

This continues until the ribosome encounters a “stop” codon and the polypeptide is released.( Called Termination)

The protein assumes its final shape and structure after interactions between amino acids and modifications by the ER or Golgi take place.

Elongation

Termination

DNA to RNA to Proteins