Genetic Chapter 10, 13, 14, 20 (sec 1)

Genetic Material

information contained in genes that gets passed onto new generation

→ source of variability among organism

Criteria for Genetic Material, molecule must be able to …

(4 things)

1. replicate

2. store information

3. express information

4. allow variation by mutation

Central Dogma of molecular genetics

DNA → RNA → PROTEIN

DNA makes RNA (transcription), which makes PROTEINS (translation)

DNA → Transcription → Translation

In 1940s, geneticists favored ____ as genetic material

proteins

Proteins and _____ acids were major candidates for genetic material

nucleic

Proteins were diverse and abundant in ____.

cells

Tetranucleotide hypothesis

• Phoebus Levene

→ DNA contains = amounts of 4 nucleotides

→ Postulated identical groups and repeats of 4 components was basis for DNA structure

→ Lack of chemical diversity inn DNA = could not store extensive g.i.

→ Proteins favored as g.i.

DNA

→ Composed of 2 strand (coiled) helical

→ Each strand is composed of subunits (nucleotides)

→ Each nucleotide consists of :

• 1 PENTOSE SUGAR (deoxyribose molecule)

• 1 PHOSPHATE GROUP

• 1 NITROGENOUS BASE

  • adenine

  • cytosine

  • guanine

  • thymine

2 Kinds of Nitrogenous Bases

1. Purines (9 member ring)

2. Pyrimidines (6 member ring)

Purines - 9 member ring

→ Adenine ; A

→ Guanine ; G

Pyrimidines - 6 member ring

→ Cytosine ; C

→ Thymine ; T

→ Uracil ; U

DNA contains ____

deoxyribose

• deoxy - without an oxygen

RNA contains _____ sugar

ribose

Bases of DNA and RNA

DNA bases ;

• A, C, T, G

RNA bases ;

• A, C, U, G

Only DNA contains T

Only RNA contains U

Nucleoside

→ Contains nitrogenous base & pentose sugar

→ molecule is composed of purine or pyrimidine base and ribose or deoxyribose sugar

Nucleotide

→ Nucleoside with phosphate group added

Phosphodiester Bonds

→ Nucleotides are linked by phosphodiester bonds between phosphate group at C - 5’ position and O H group on C - 3’ position

Nucleoside Monophosphates ; NMP

a nucleotide

Nucleoside Diphosphate ; NDP

Nucleotide with addition of 2 phosphate groups

Nucleoside Triphosphate ; NTP

Nucleotide with addition of 3 phosphate groups

Triphosphate

→ Serve as precursor molecule during nucleic acid synthesis

ATP and GTP

→ Adenosine triphosphate and guanosine triphosphate

→ large amount of energy involved in adding/removing terminal phosphate group

Watson and Crick 1953

Proposed the structure of DNA as a double helix

Chargaff 1949-1953

→ Proposed base composition

→ Amount of A is proportional to T

→ Amount of C is proportional to G

→ Percentage of C + G does not equal percentage of A + T

Base composition analysis (Chargaff) and X-ray diffraction provided crucial data to _____ and Crick

Watson

X-ray Diffraction

→ studies by Rosalind Franklin 50-53 showed DNA had a 3.4 angstrom periodicity, characteristic of helical structure

Watson and Crick Model of DNA :

→ Double helix

→ 2 anti-parallel strands connected by base pairing

→ Stacked nitrogenous bases

Base Pairing --- Hydrogen Bonds

→ Chemical affinity produces hydrogen bonds in pair of bases

• A-T and G-C base provides complementarity of 2 strands and chemical stability to the helix

• A-T ; Double bond

• G-C ; Triple bond

Watson and Crick : Semiconservative Model

→ Storage of genetic information in sequence of bases

→ Mutations or genetic changes that could result in alteration of bases

Nucleotide Bonding

→ Each nucleotide is bound to a nucleotide on the other chain by weak hydrogen bonds between specific pairs of bases

→ A pairs w/ T

→ G pairs w/ C

→ 2 chains are complementary w/ opposite polarities

Genetic Code : Translation

→ involves the synthesis of proteins consisting of a chain of amino acids whose sequence id specified by the coding information in mRNA

• mRNA carries the “genetic code” = chemical info. originating in DNA which specifics the primary structure of proteins

Translation of mRNA

→ Biological polymerization of amino acids into polypeptide chains

Translation requires : (4)

1. Amino acids

2. mRNA

3. tRNA

4. Ribosomes

tRNAs

→ adapt genetic information present as specific triplet codons in mRNA to corresponding amino acid

→ tRNA anticodons complement mRNAs

→ tRNAs carry corresponding amino acids

Ribosomes

→ Essential role in expression of genetic information

→ Consist of ribosomal proteins and ribosomal RNAs

→ Consists of large and small subunits

• Prokaryote ribosomes are 70s

• Eukaryote ribosome are 80s

tRNAs characteristics

→ small in size and very stable

→ 75-90n nucleotides

→ transcribed from DNA

→ Contain posttranscriptional modified bases

• important for hydrogen bonding

• confer structural stability

→ tRNAs have a cloverleaf structure

Anticodon

→ tRNA has anticodon that complementarily base-pairs w/ codon in mRNA

→ Corresponding amino acid is covalently linked to CCA sequence at 3’ end of all tRNAs

Translation

→ tRNAs are covalently attached to specific amino acids and contain anti-codon complementary to the mRNA codon

→ Base pairing between the tRNA anti-codon and the mRNA codon on the ribosome places amino acids in the correct linear sequence in translation

Translation of mRNA divides into 3 steps :

1. Initiation

2. Elongation

3. Termination

Initiation of Translation requires :

1. Small and large ribosomal subunits

2. mRNA molecule

3. GTP

4. Charged initiator tRNA

5. Mg^2+

6. Initiation factors

Elongation

→ Both ribosomal subunits assembled w/ mRNA

→ Forms P site and A site

Termination

→ signaled by stop codons (UAG, UAA, UGA) in A site

→ Codons do not specify any amino acid

GTP-dependent release factors

→ Stimulates hydrolysis of polypeptide from peptidyl tRNA - released from translation complex

Charging tRNA

→ Aminoacylation : tRNA charging

• before translation can proceed, tRNA molecules must be chemically linked to respective amino acids

• Aminoacyl tRNA synthetase

  • enzyme that catalyzes aminoacylation

→ 20 different synthetases, 1 for each amino acid

→ Highly specific; recognize only 1 amino acid

Gene Expression Principles

→ Gene expression involves processes of transcription and translation which result in the production of proteins whose structure is determined by genes