Deoxyribonucleic Acid

The Discovery of Genetic Material

Frederick Griffith

injected mice with different strains of bacteria

named the transforming factor

Oswald Avery

demonstrated the transforming factor to be DNA

Alfred Hershey and Martha Chase

used bacteriophages to demonstrate that the genetic material is DNA, not proteins

Chargaff’s Rule

purines pair with pyrimidines

adenine pairs with thymine

guanine pairs with cytosine

Rosalind Franklin and Maurice Wilkins

took X-ray diffraction pictures of DNA which showed the double helix nature of DNA

James Watson and Francis Crick

proposed the double helix model of DNA in 1953

A polymer composed of two long chains of nucleotides (the building blocks of DNA)

five carbon sugar

deoxyribose in DNA

ribose in RNA

phosphate group

nitrogen base

Two types of nitrogen bases

purines

adenine

guanine

pyrimidines

thymine

cytosine

Adenine ALWAYS pairs with Thymine in DNA.

Guanine ALWAYS pairs with Cytosine.

DNA twists around a central axis to form a double helix.

Because of Base Pairing:

rungs of the ladder are even

each strand is complementary to the other strand

Replication of DNA

 

duplication of DNA

Helicase “unzips” the two strands. the unpaired bases in each strand react with complementary bases of nucleotides floating freely in nucleus.

Polymerase connects the sugar and phosphate groups.

Semi-conservative replication: two new DNA molecules, identical to the first. each consists of one old strand and one new strand.

Replication occurs at many different places along the molecule simultaneously.

Enzymes are used to recognize and “proofread” DNA to make sure it is copied correctly and to repair DNA to prevent mutations.

RNA functions primarily in directing protein synthesis.

RNA differs from DNA in three ways:

only one strand of nucleotides

its sugar is ribose instead of deoxyribose

it has the nitrogen base uracil instead of thymine

Types of RNA

mRNA (messenger RNA): a single, uncoiled strand which transmits information from DNA for use in protein synthesis. it serves as the template for the assembly of amino acids during protein synthesis

tRNA (transfer RNA): a single strand folded back on itself like a hairpin. this due to partial base pairing within the same strand. each of its 20+ varieties bonds only with one amino acid

rRNA (ribosomal RNA): found in a globular form. it is the major component of ribosomes

genetic code: system that contains information needed by cells for proper functioning.

the genetic code is built into the arrangement of nitrogen bases in a particular sequence of DNA

therefore, DNA contains the information needed to put the amino acids together in the proper sequence

Codon: a specific group of three sequential bases of mRNA. it recognizes and codes for a specific amino acid.

each mRNA codon attracts a group of bases on tRNA; the tRNA has a specific amino acid attached to it

AUG: universal “start” codon

UAA, UAG, UGA: stop codons

The production of RNA from DNA

RNA Polymerase binds to DNA and separates the complementary strands

the enzyme directs bonds to form between bases of DNA and complementary bases of RNA floating freely in the nucleus

next, it bonds the sugar to the phosphate.

the enzyme releases the RNA strand when it reaches the stop codon.

The assembling of proteins

mRNA moves out of the nucleus and migrates to the ribosomes

amino acids are transported to the ribosomes by tRNA

anticodon found in tRNA is complementary to the codon of mRNA

assembly begins when ribosome attaches to AUG (methionine) codon on mRNA

Translation

AUG on mRNA pairs with UAC on tRNA.

the ribosome moves along the mRNA translating it

each amino acid is linked to the chain by an enzyme

the process continues until the ribosome reaches a stop codon

the ribosome is released and the polypeptide is complete

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