How Many Scientists Does it Take to Discover a Molecule?

Erwin Chargaff (Born 1905) Russian

Moved to the U.S. Discovered that there could not be

only one base attached There was always an equal number of

A’s to T’s and C’s to G’s. There could be different amounts of

these two groupings.

Rosalind Franklin (1920 – 1958) British

Worked under Maurice Wilkins Used X-ray diffraction to get images

of DNA Her work was passed along to

Watson and Crick by Wilkins. She did not get recognized for her

work possible because she was a woman in science when it was not fully acceptable

Watson and Crick

Crick (Born 1916) British Watson (Born 1928) American

Used images and papers produced by Franklin

Discovered double helix shape They got the Nobel Prize in 1953 along with Wilkins

DNA Replication

DNA replicates when the cells divide and when the sex cells are formed

Groups of proteins guide the process, but DNA polymerase is the major functioning enzyme.

Three major events to begin DNA replication

1. The two nucleotide chains must separate forming a replication fork.

A. The chains are separated by enzymes called helicases

2. DNA polymerase binds to the separated chains of DNA and assembles complimentary nucleotides to the old strand

3. The new strand of DNA hydrogen bonds with the old strand and winds back to form a helix

Checking the System

Incoming nucleotides bind to the DNA polymerase and the polymerase checks to see if the incoming nucleotide pairs correctly with the DNA template.

Error rate is about 1 error in 10000 base pairs (108)

Direction of Replication

Replication takes place in both directions.

DNA polymerase begins replication simultaneously at many points along the separated nucleotide.

The Other Nucleic Acid

Remember that: DNA is located in the nucleus and under no

circumstances can leave the nucleus.

Proteins are manufactured in the cytoplasm at the ribsomes.

DNA is the instructions for making those proteins.

RNA: Ribonucleic Acid

Single Stranded Made of nucleotides

Sugar: Ribose Phosphate group Nitrogen bases

The nitrogen bases in RNA have one difference.

Thymine is replace by uracil So A pairs to U; G pairs to C

Three Types of RNA

Messenger RNA (mRNA) Carries the code from DNA to the

ribosome Transfer RNA (tRNA)

Picks up individual amino acids in the cytoplasm and carries them to the ribosome to be delivered

Ribosomal RNA (rRNA) Helps bind mRNA and tRNA

together during protein synthesis

RNA Transcription

DNA transcribes a copy of mRNA in the nucleus of the cell. The mRNA takes the DNA code out of the nucleus and to the ribosomes.

Steps of Transcription

1. RNA polymerase makes RNA copies of specific segments of DNA.

This begins at a promotor site on DNA

Only the DNA template is used for transcription

Steps of Transcription

2. RNA polymerase adds complimentary RNA nucleotides to form a new RNA molecule.

• Complimentary base pairing determines the nucleotide sequence

Steps of Transcription

3. Transcription ends at the DNA terminator. Both DNA and RNA are released by the RNA polymerase.

All three types of RNA molecules are transcribed by this process.

Protein Synthesis – Structure and Composition

Proteins are polymers made of amino acids bonded by peptide bonds

Proteins are made up of specific sequences of amino acids There are 20 kinds of amino acids

Protein assembly

Codon (AUG) means “Start” or can code for the amino acid methinone

MRNA codon is paired with tRNA anticodon Amino acids are bonded with peptide bonds

and the polypeptide chain grows into a protein At the stop codon (UAA, UAG, UGA) translation

ends and the protein is finished The protein folds up Several ribosomes can translate the same

mRNA transcript at the same time

Mutations

A change in the nucleotide sequence is one spot