Nucleic Acids and Protein Synthesis 
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Cell   à   Nucleus    à    Chromosomes   à   Genes    à     DNA 

Proteins

• Organic molecules (macromolecules) made by cells
• Make up a large part of your body
• Used for growth, repair, enzymes, etc.
• Composed of long chains of small units called amino acids bonded together by peptide bonds
• Twenty amino acids exist

DNA

• Deoxyribonucleic acid is a coiled double helix carrying hereditary information of the cell

• Contains the instructions for making proteins from 20 different amino acids
• Appears as chromatin when cell not dividing

• Structure discovered by Watson & Crick in 1953
• Sides made of pentose (5-sided) sugars attached to phosphate groups by phosphodiester bonds
• Pentose sugar called Deoxyribose

• Steps or rungs of DNA made of 4 nitrogen-containing bases held together by weak hydrogen bonds
• Purines (double carbon-nitrogen rings) include adenine (A) and guanine (G)
• Pyrimidines (single carbon-nitrogen rings) include thymine (T) and cytosine (C)

• Base pairing means a purine bonds to a pyrimidine   (Example:  A --- T   and   C --- G)
• Coiled, double stranded molecule known as double helix
• Make up chromosomes in the nucleus
• Subunits of DNA called nucleotides
• Nucleotides contain a phosphate, a Deoxyribose sugar, and one nitrogen base (A,T,C, or G)

• Free nucleotides also exist in nucleus
• Most DNA is coiled or twisted to the right
• Left twisted DNA is called southpaw or Z-DNA
• Hot spots which can result in mutations occur where right & left twisted DNA meet

History of DNA discovery

• Freidrich Miescher (1868) found nuclear material to be ½ protein & ½ unknown substance
• 1890’s, unknown nuclear substance named DNA
• Walter Sutton (1902) discovered DNA in chromosomes
• Fredrick Griffith (1928) working with Streptococcus pneumoniae conducted transformation experiments of virulent & nonvirulent bacterial strains
• Levene (1920’s) determined 3 parts of a nucleotide
• Hershey & Chase (1952) used bacteriophages (viruses) to show that DNA, not protein, was the cell’s hereditary material
• Rosalind Franklin (early 1950’s) used x-rays to photograph DNA crystals

• Erwin Chargraff (1950’s) determined that the amount of A=T and amount of C=G in DNA; called Chargaff’s Rule
• Watson & Crick discovered double helix shape of DNA & built the 1^st model

 DNA Replication

•  Process by which DNA makes a copy of itself

• Occurs during S phase of interphase before cell division

• Extremely rapid and accurate (only 1 in a billion are incorrectly paired)

• Requires many enzymes & ATP (energy)

• Begins at special sites along DNA called origins of replication where 2 strands open & separate making  a replication fork

• Nucleotides added & new strand forms at replication forks

• DNA helicase (enzyme) uncoils & breaks the weak hydrogen bonds between complementary bases (strands separate)

•  DNA polymerase adds new nucleotides to the exposed bases in the 5’ to 3’ direction

•  Leading strand (built toward replication fork) completed in one piece

• Lagging strand (built moving away from the replication fork) is made in sections called Okazaki fragments

OKAZAKI FRAGMENTS

•  DNA ligase helps join Okazaki segments together

• DNA polymerase proofreads the new DNA checking for errors & repairing them; called excision repair

• Helicase recoils the two, new identical DNA molecules

RNA

• Ribonucleic acid
• Single stranded molecule  

• Found in nucleus & cytoplasm
• Contains ribose sugar
• Contains the nitrogen base uracil (U) instead of thymine so A pairs with U
• Base pairings are A-U and C-G
• Three types of RNA exist (mRNA, TRNA, & rRNA)

 mRNA

• Messenger RNA
• Single, uncoiled, straight strand of nucleic acid
• Found in the nucleus & cytoplasm
• Copies DNA’s instructions & carries them to the ribosomes where proteins can be made
• mRNA’s base sequence is translated into the amino acid sequence of a protein
• Three consecutive bases on mRNA called a codon (e.g. UAA, CGC, AGU)
• Reusable

tRNA

• Transfer RNA
• Single stranded molecule containing 80 nucleotides in the shape of a cloverleaf
• Carries amino acids in the cytoplasm to ribosomes for protein assembly
• Three bases on tRNA that are complementary to a codon on mRNA are called anticodons (e.g. codon- UUA; anticodon- AAU)
• Amino Acid attachment site across from anticodon site on tRNA
• Enters a ribosome & reads mRNA codons and links together correct sequence of amino acids to make a protein
• Reusable  

rRNA

• Ribosomal RNA
• Globular shape
• Helps make up the structure of the ribosomes  
• rRNA & protein make up the large & small subunits of ribosomes
• Ribosomes are the site of translation (making polypeptides)

• Aids in moving ribosomes along the mRNA strand as amino acids are linked together to make a protein

 Amino Acids

• 20 exist
• Linked together in a process called protein synthesis in the cytoplasm to make polypeptides (subunits of proteins)
• DNA contains the instructions for making proteins but is too large to leave the nucleus
• Three consecutive bases on DNA called a triplet (e.g. TCG, ATG, ATT)
• mRNA codon table tells what 3 bases on mRNA code for each amino acid (64 combinations of 3 bases)
• Methionine (AUG) on mRNA is called the start codon because it triggers the linking of amino acids
• UAA, UGA,  & UAG on mRNA signal ribosomes to stop linking amino acids together

Genetic Code (RNA)

  Practice Table:

 Protein Synthesis

• Consists of 2 parts --- Transcription & Translation
• Begins in the nucleus with mRNA copying DNA’s instructions for proteins (transcription)
• Completed in the cytoplasm when tRNA enters ribosomes to read mRNA codons and link together amino acids (translation)

 Steps in Transcription

1. DNA helicase (enzyme) uncoils the DNA molecule
2. RNA polymerase  (enzyme) binds to a region of DNA called the promoter which has the start codon TAC to code for the amino acid methionine
3. Promoters mark the beginning of a DNA chain in prokaryotes, but mark the beginning of 1 to several related genes in eukaryotes
4. The 2 DNA strands separate, but only one will serve as the template & be copied
5. Free nucleotides are joined to the template by RNA polymerase in the 5’ to 3’ direction to form the mRNA strand
6. mRNA sequence is built until the enzyme reaches an area on DNA called the termination signal
7. RNA polymerase breaks loose from DNA and the newly made mRNA
8. Eukaryotic mRNA is modified (unneeded sections snipped out by enzymes & rejoined) before leaving the nucleus through nuclear pores, but prokaryotic RNA isn’t
9. All 3 types of RNA called transcripts are produced by this method

 Steps in Translation

1. mRNA brings the copied DNA code from the nucleus to the cytoplasm
2. mRNA attaches to one end of a ribosome; called initiation
3. tRNA’s attach the correct amino acid floating in the cytoplasm to themselves
4. tRNA with its attached amino acid have 2 binding sites where they join the ribosome
5. The tRNA anticodon “reads” & temporarily attaches to the mRNA codon in the ribosome
6. Two amino acids at a time are linked together by peptide bonds to make polypeptide -chains (protein subunits); called elongation
7. Ribosomes) move along the mRNA strand until they reach a stop codon (UAA, UGA, or UAG); called termination

8. tRNA’s break loose from amino acid, leave the ribosome, & return to cytoplasm to pick up another amino acid

Click here for an animation of Translation