CHAPTER 10: DNA,RNA & Protein Synthesis

I. Discovery of DNA

• Scientist originally believed PROTEINS would be the molecules which contained hereditary information.

• Some scientists who did experiments that proved DNA had genetic information:– 1. Fredrick Griffith– 2. Oswald Avery– 3. Hershey & Chase

4. James Watson & Francis Crick

• In 1953 DNA structure discovered• Double Helix model.

– (2 chains of DNA)– Showed how DNA could replicate.

• Relied on work of other scientists:

– Rosalind Franklin & Maurice Wilkins took X-ray photos of DNA structure

– ( Franklin died 1958 before Watson & Crick received Nobel Prize. 1962.)

II. DNA structure

A. Double Helix shape is formed by base pairs attached to a sugar-phosphate backbone.

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B. Parts of Nucleotides

1. 5 carbon sugar in DNA- deoxyribose(in RNA= ribose)

2. phosphate group3. nitrogenous bases ( there are 4 different ones)• Adenine• Guanine• Cytosine• Thymine (in RNA- no thymine- Uracil is the base)

C. How Chemical Bonds hold DNA together

• Covalent bonds- between sugar & phosphates of 2 nucleotides

• Hydrogen bonds- between complementary nitrogenous bases –

www.molecularstation.com/dna/dna-structure/

D. Base Pairing Rules

• Adenine always bonds with Thymine (AT)

• Guanine always bonds with Cytosine (GC)

• Note: 3 hydrogen bonds: GC– 2 Hydrogen bonds: AT

Must have 1 purine (a 2 ring shape) plus 1 pyriomidine (1 ring shape) in each pair or the would not “fit” inside ladder of DNA

• 2. Purines (A, G) double C ring

• 3. Pyrimidines (T, C) single C ring

III. DNA Replication

A. Is the process by which DNA is copied in a cell before a cell divides by mitosis, meiosis or binary fission.

B. Steps in Replication1. Helicase enzymes -separate

hydrogen bonds in strands – create replication fork

2. Attach- DNA polymerase enzyme -adds nucleotides

3. Release –DNA polymerase enzyme – now have 2 identical DNA strands

oak.cats.ohiou.edu/.../Heredity/Heredity.htm

C. Errors • DNA replication is very accurate.• Errors occur ~ 1 in 1 billion paired nucleotides.• “Proofreading” enzyme checks for “spelling” errors. *If a mistake does occur- new DNA is different:

• Mutation- a change in the nucleotide sequence of a DNA molecule.

• Caused by chemicals, radiation,UV rays.• Mutations can be favorable • - or harmful. (example- cancer)

IV Protein SynthesisA. Flow of Information (DNA – RNA - Proteins)

Before protein can be synthesized, the instructions in DNA must first be copied to another type of nucleic acid called messenger RNA.

Then -a group of 3 nucleic acids codes for an amino acid & it is built at the ribosomal RNA with help from the transfer RNA

RNA differs from DNA in the following ways:

• RNA is single stranded while DNA is double stranded.

• RNA has a sugar called ribose while DNA has a sugar called deoxyribose.

• RNA has the nitrogenous base uracil while DNA has the base thymine.

B. 3 types RNA:1. messenger RNA(mRNA)

2. transfer RNA (tRNA)

3. ribosomal RNA (rRNA)

1. Messenger RNA, or mRNA.carries the code for building a protein from thenucleus to the ribosomes in the cytoplasm. It actsas a messenger.

2. Transfer RNA or tRNA. picks up specific amino acids in the cytoplasm &brings them into position on ribosome where they are joined together in specific order tomake a specific protein.

3. Ribosomal RNA or rRNA place for protein synthesis

C. Steps in Transcription-making RNA

1. RNA polymerase (enzyme)– binds to promoter area on DNA

2. Nucleotides added & joined by the enzyme (RNA polymerase)

3. Termination signal- stop- RNA polymerase releases both DNA & new RNA molecules

www.smartdraw.com/.../dna_transcription.htm

D. Proteins• Review of protein structure

-recall that proteins are made of amino acids joined together with peptide bonds-there are 20 different amino acids, the order they are joined determines the structure & function of the proteins.-proteins can be very large, complicated molecules

mRNA codons for specific amino acids

• Each 3 nucleotide sequence (letters) in mRNA encodes for 1 specific amino acid, or a “start” or “stop” signal.

• Each 3 nucleotide group is called a codon.• The genetic code- means the rules that

relate how a particular sequence of nitrogenous bases corresponds to a particular amino acid.

teachline.ls.huji.ac.il/72693/weekly2.html

E. Steps in Translation

1. Initiation- tRNA & mRNA join together.(The codon is on the mRNA, the “anticodon” is on tRNA)The tRNA has an amino acid attached to it)

2. Elongation- continued as ribosome moves the distance of 1 codon on mRNA

3. Elongation is built with new tRNAs attaching each amino acid as it reads the codons on the mRNA.

4. Termination- ribosome reaches “stop” codon on the mRNA

5. Disassembly – each piece is free.

(see sequence page 208-209 in textbook)

Remember:

• Replication- copying DNA from DNA

• Transcription- making RNA from DNA

• Translation- making proteins