DNA The molecule of Life

Nucleic Acids Ribonucleic acid (RNA) Single stranded Builds proteins Acts as enzymes Three types mRNA tRNA rRNA

DNA DNA is often called the blueprint of life Nucleic acids store and transmit hereditary information In simple terms it contains the instructions for making proteins in the cell.

DNA by the numbers Each cell has about 2 meters of DNA. The average human has 75 trillion cells. The average human has enough DNA to go from the earth to the sun more than 400 times. DNA has a diameter of only 0.000000002 meter. The earth is 150 billion meters or 93 million miles from the sun.

The Shape of the Molecule DNA is a very long polymer. The basic shape is like a twisted ladder or spiral staircase. This is called a double helix.

DNA STRUCTURE Adenosine Mono Phosphate (AMP) OH HO Phosphate NH2 N Base - H+ OH O CH2 Sugar H 2’ 3’ 4’ 5’ 1’ Nucleotide OH

DNA Structure Sugar Base OH O CH2 H P O OH HO Phosphate The outer parts of the ladder are made of sugars and phosphates. The steps of the ladder are made of pairs of nitrogen bases. NH2 N Base

Four Nitrogenous Bases DNA Four Nitrogenous Bases Cytosine C Thymine T Adenine A Guanine G

One Strand of DNA phosphate nucleotide base sugar One strand of DNA is a polymer of nucleotides. One strand of DNA has many millions of nucleotides. The backbone of the molecule is alternating phosphate and deoxyribose, a sugar. The teeth are nitrogenous bases.

Two Types of Bases in DNA Purines are double ring bases. N Adenine NH2 NH2 O N NH Guanine

Two Kinds of Bases in DNA NH2 Cytosine Pyrimidines are single ring bases. Uracil (RNA) CH3 N O NH Thymine (DNA) Uracil is found in RNA (more later)

Nitrogenous bases Pyrimidines Nucleotide Bases CH Uracil (in RNA) U Ribose (in RNA) Nitrogenous bases Pyrimidines C N O H NH2 HN CH3 Cytosine Thymine (in DNA) T HC NH Adenine A Guanine G Purines HOCH2 OH Pentose sugars Deoxyribose (in DNA) 4’ 5” 3’ 2’ 1’ Pyrimidines C = Cytosine T = Thymine U = Uracil Purines A = Adenine G = Guanine

Two Stranded DNA Remember, DNA has two strands that fit together something like a zipper. The teeth are the nitrogenous bases but why do they stick together?

Hydrogen Bonds + - - + CH3 H Thymine Adenine N N H O The bases attract each other because of hydrogen bonds. Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA. CH3 N O H + - Thymine N H - + Adenine

Chargaff’s Rule Adenine must pair with Thymine Guanine must pair with Cytosine Their amounts in a given DNA molecule will be about the same. T A G C

The Code of Life… A T C G T A T G C G G… The “code” of the chromosome is the SPECIFIC ORDER in which the bases occur. A T C G T A T G C G G…

DNA Structure Determination Edwin Chargaff 1949 biochemist Showed that the amount of A=T and G=C Wilkins and Franklin 1952 X-ray picture of DNA Watson and Crick 1953 proposed a 3-D model accounting for the data.

The Watson - Crick Model Of DNA T A G C C G G C T A The Watson - Crick Model Of DNA 3.4 nm 1 nm Minor groove Major groove 0.34 nm

DNA Replication The preparation of exact copies of a molecule. DNA replication is essential for life It allows organisms to create copies of themselves. DNA is wrapped tightly around histones and coiled tightly to form chromosomes

Why is accuracy of Nucleotide sequence in the DNA of a gene important? Nucleotide sequence in messenger RNA (mRNA) Transcription Nucleotide sequence in a protein (tRNA) Translation Structure and function of the protein Form and function Characteristics or traits (normal health or problems)

DNA Replication A-T G-C DNA must be copied to produce 2 IDENTICAL new complementary strands following the rules of base pairing: A-T G-C Each strand of the original DNA serves as a template for the new strand

DNA Replication Semi-conservative Model: Meselson and Stahl showed the two strands of the parental molecule separate, and each functions as a template for synthesis of a new complementary strand. DNA Template Parental DNA New DNA

Uses energy from ATP to unwind the DNA by breaking the Hydrogen bonds SSB Areas where the DNA separates are called replication forks

Schematic representation of DNA Polymerase Structure resembles a human right hand Template DNA thread through the palm Thumb and fingers wrapped around the DNA Polymerase has “proofreading” role Backtrack and correct errors Only about one error per billion nucleotides

Multiple Replication Forks During Eukaryotic DNA Synthesis