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2 DNA DNA.DNA is often called the blueprint of life. Enables cells to have different forms and functions.Enables cells to have different forms and functions. DNA contains the instructions for making proteins within the cell.

3 DNA by the Numbers Each cell has about 2 m 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 m. The earth is 150 billion m or 93 million miles from the sun.

4 DNA Why do we study DNA? We study DNA for many reasons, e.g., its central importance to all life on Earth, medical benefits such as cures for diseases, better food crops.

5 Chromosomes and DNA Our genes are on our chromosomes. Chromosomes are made up of a chemical called DNA.

6 Structure of DNA Nucleic acid is an organic compound. DNA is constructed of repeating subunits called nucleotides. Each DNA molecule consists of 2 long chains of nucleotides.

7 3 Essential Parts of a DNA nucleotide. 1) Deoxyribose – The sugar molecule 2 )Nitrogen containing base 3 )A phosphate group – a single atom of phosphorus surrounded by oxygen atoms

8 Nucleotides CC C O Phosphate O C C O -P O O O O O O O One deoxyribose together with its phosphate and base make a nucleotide. Nitrogenous base Deoxyribose

9 The Nucleotide The deoxyribose and the phosphate group are identical in all DNA nucleotides. The nitrogen containing bases may be any 1 of 4 different kinds.

10 4 Nitrogen Bases NucleotideAbbreviation AdenineA GuanineG CytosineC ThymineT

11 Nitrogen Containing Bases Nitrogen containing bases fall into 2 distinct categories. –Pyrimidines - Bases that have 1 ring of carbon and nitrogen atoms. –Purines – Bases that have 2 rings of carbon and nitrogen atoms.

12 Two Kinds of Bases in DNA C C C C N N O N C C C C N N N N N C PyrimidinePurine ThymineGuanine CytosineAdenine Pyrimidine Purine

13 Thymine and Cytosine are pyrimidines Thymine and cytosine each have one ring of carbon and nitrogen atoms.Thymine and cytosine each have one ring of carbon and nitrogen atoms. C C C C N N O N cytosine C C C C N N O O thymine C

14 Adenine and Guanine are purines Adenine and guanine each have two rings of carbon and nitrogen atoms.Adenine and guanine each have two rings of carbon and nitrogen atoms. C C C C N N N Adenine N N C C C C C N N O N Guanine N N C

15 Watson & Crick’s Model

16 Watson and Crick In 1953 suggested a model for the structure of DNA. The model consists of two nucleotide chains that wrap around each other to form a double spiral. The model was inspired by an x-ray photo of DNA crystals.

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18 The Shape of the Molecule This is called a double helix.

19 One Strand of DNA The backbone of the molecule is alternating phosphates and deoxyribose sugar The teeth are nitrogenous bases. phosphate deoxyribose bases

20 One Strand of DNA One strand of DNA is a polymer of nucleotides. One strand of DNA has many millions of nucleotides. nucleotide

21 Two Stranded DNA The teeth fit together like a zipper. The teeth are the nitrogenous bases but why do they stick together?

22 C C C C N N O N C C C C N N O N N N C Hydrogen Bonds 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. The bonds between cytosine and guanine are shown here with dotted lines

23 Hydrogen Bonds, Hydrogen Bonds, cont. When making hydrogen bonds, cytosine always pairs up with guanine Adenine always pairs up with thymine Adenine is bonded to thymine here C C C C N N N N N C C C C C N N O O C

24 Base-pairing rules Adenine and Thymine always join together A T Cytosine and Guanine always join together C G

25 Base Pairing Cytosine and Guanine form 3 hydrogen bonds. Adenine and Thymine form 2 hydrogen bonds.

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27 Replication of DNA During which phase of the cell cycle does replication occur? Interphase

28 Replication The process of copying DNA in a cell is called Replication. Two nucleotide chains separate by unwinding and each chain serves as a template for a new nucleotide chain.

29 DNA Replication The 1 st step is the separation of the nucleotide chains. The chains are separated by enzymes called helicases. The enzyme moves down the chain breaking hydrogen bonds. The point at which the chains separate is called the replication fork

30 DNA Replication Strands unwinding and new strands forming. Where is the fork?

31 DNA Replication DNA polymerases bind to the separated chains of DNA. The polymerases move along the separate strands of DNA assembling new complementary strands.

32 DNA Replication What would the complementary strand of DNA be for T-T-C-A-G- T-C-G-A? A-A-G-T-C-A-G-C-T

33 DNA Replication DNA replication doesn’t start at one end of the molecule and end at the other. WHY? DNA polymerases begin replication at multiple sites in the molecule allowing for FASTER replication. Example: Replication in a fruit fly starts at almost 6000 sites simultaneously.

34 DNA Replication The end result is two exact copies of the original DNA molecule. The cell is now ready to undergo division.

35 Accuracy and Repair How often in the process of DNA replication does an error occur? Answer: Approximately 1 error in every 10,000 pairs of nucleotides. Even one error, also known as a mutation can have a serious effect in the new cell.

36 Accuracy and Repair Enzymes that proofread and repair the strands bring the error rate to approximately 1 per 1 billion nucleotides. DNA may also be damaged by chemicals and UV radiation from the sun.