Nucleic Acids and Protein Synthesis

10 – 1 DNA10 – 2 RNA

10 – 3 Protein Synthesis

DNA

• Ultimate source of genetic diversity is Deoxyribonucleic Acid (DNA)

• Primary function of DNA– Store and transmit the genetic information that

tells cells which proteins to make and when to make them.

– Proteins in turn form the structural units of cells and help control chemical processes within cells.

Structure of DNA

• DNA is an organic compound• Made up of repeating subunits called

nucleotides• Each DNA molecule consists of two long

chains of nucleotides• Nucleotide made up of three parts– Sugar molecule called Deoxyribose– Phosphate group– Nitrogen containing base

• Four different nitrogen bases– Adenine - A– Guanine - G– Cytosine - C– Thymine – T

• Purines * Pyrimidines– Adenine and Guanine - Thymine and Cytosine

The Double Helix

• Discovered in 1953 by James Watson and Francis Crick.

• Model said that DNA is composed of two nucleotide chains that wrap around each other to form a double spiral – Double Helix

• Model was inspired in part by X-ray photographs of DNA crystals.

• The model provided an explanation for how copies of DNA could be made and how genetic information might be stored and used within cells.

• Received Nobel Prize in 1962.

• Covalent bonds hold phosphate and sugar together.

• Hydrogen bonds hold Nitrogen bases together.

• Form Twisted ladder.• Phosphate and sugar make sides of ladder,

and nitrogen bases make rungs of ladder

• Rungs of ladder are all of uniform length. • Each rung is made up of 1 purine and 1

pyrimidine. This makes sure every rung is the same length.

• Double helix has a right handed twist• Each full turn is made up of ten base pairs.

Complementary Base Pairs

• Adenine bonds with Thymine• Guanine bonds with Cytosine• Complementary base pairs led to suggestions

of how DNA might copy itself.

Replication of DNA

• Replication: process of copying DNA• Step 1: Separation of two nucleotide chains. • Point at which they separate: called

replication fork.• Chains are separated by enzymes called

helicases, which break hydrogen bonds and chains separate.

• Step 2: DNA polymerase bind to the separated chains of DNA.

• New chains of DNA are assembled using nucleotides in the surrounding medium that are complementary to the existing DNA chains.

• Step 3: Nucleotides are joined to the new chains by covalent bonds between deoxyribose and phosphate groups.

• They are joined to the original nucleotide chain by hydrogen bonds.

• DNA replication does not begin at one end and work its way to the other.

• DNA polymerase begin replication simultaneously at many points along the separated nucleotide chains.

• When replication is completed, two new exact copies of the original DNA molecule are produced.

Accuracy and Repair

• 1 error in every 10,000 paired nucleotides• Mutation: a change in the nucleotide

sequence at even one location.• Can have very serious effects in new cells.• DNA proofreading and repair helps keep the

error rate to one error per 1 billion nucleotides.

• Enzymes proofread and make repairs

RNA

• Nucleic Acid responsible for the movement of genetic information form the DNA in the nucleus to the site of protein synthesis in the cytosol.

Structure of RNA

• A nucleic acid made up of repeating nucleotides.

• Difference between DNA and RNA.– Sugar molecule is Ribose – No Thymine, Uracil instead– Single strand vs. double strand

Types of RNA• Three types of RNA– Messenger RNA (mRNA) – consists of RNA nucleotides

in the form of a single uncoiled chain. mRNA carries genetic information from the DNA in the nucleus to the cytosol .

– Transfer RNA (tRNA) – consists of a single chain of about 80 RNA nucleotides folded into a hairpin shape that binds to specific amino acids.

– Ribosomal RNA (rRNA) – the most abundant form of RNA. rRNA consist of RNA nucleotides in a globular form. Joined by proteins, rRNA makes up the ribosomes where proteins are made.

Transcription

• Process by which genetic information is copied from DNA to RNA.

• Steps of transcription– 1. RNA polymerase, enzyme, synthesizes RNA

copies of specific sequences of DNA– RNA polymerase initiates RNA transcription by

binding to the specific regions of DNA called promoters.

– Promoter marks the beginning of the DNA chain that will be transcribed

• When RNA polymerase binds to a promoter, the DNA molecule in that region separates.

• Only one of the separated DNA chains, called the template is used for transcription

• RNA polymerase attaches to the first DNA nucleotide of the template chain. Then it begins adding complementary RNA nucleotides to the newly forming RNA molecule

• Transcription continues one nucleotide at a time until the RNA polymerase reaches a DNA region called the termination signal.

Products of Transcription

• Called transcripts• mRNA, tRNA, rRNA• mRNA moves through the pores of the nuclear

membrane into the cytosol of the cell, where it will direct the synthesis of proteins.

Protein Synthesis

• The production of proteins.• The amount and kind of proteins that are

produced in a cell determine the structure and function of the cell.

• Proteins carry out the genetic instructions encoded in an organisms DNA

Protein Structure and Composition

• Proteins are polymers• Made up of one or more polypeptides, each of

which consists of a specific sequence of amino acids linked together by peptide bonds.

• Polypeptides that make up one protein may consist of hundreds or thousands of the 20 different amino acids determines how the polypeptides will twist and fold into the three dimensional structure of the protein.

• The function of a protein depends on its ability to bind with other molecules within a cell.

• The function depends on the protein’s three dimensional structure, which is determined by its amino acid sequence.

The Genetic Code

• The sequence of nucleotides in an mRNA transcript is translated into a sequence of amino acids.

• The genetic information necessary for making proteins is encoded in series of three mRNA nucleotides – called codon.

• Each codon codes for a specific amino acid.

Translation

• Process of assembling polypeptides from information encoded in mRNA

• The process of translation begins when mRNA leaves the nucleus through pores in the nuclear membrane.

• The mRNA then migrates to a ribosome in the cytosol, the site of protein synthesis

tRNA and Anticodons

• Amino acids from cytosol are transported to the ribosomes by tRNA molecules.

• A tRNA molecule has a region that bonds to a specific amino acid. The loop opposite the site of amino acid attachment bears a sequence of three nucleotides called an anticodon

• The tRNA anticodon is complementary to and pairs with its corresponding mRNA codon.

Protein Assembly

• Amino acids are linked together by a peptide bond (covalent)