DNA All life on earth uses a chemical called DNA to carry its genetic code or blueprint. In this lesson we be examining the structure of this unique molecule. {Point out the alligator’s eyes in the first picture.} By the way, can you make out what this is? *************************************************************** [The goal of this presentation is to introduce high school biology students to the chemical structure of DNA. It is meant to be presented in the classroom while accompanying the teacher’s lecture, under the control of the teacher.]

DNA is the code for the making of proteins used for structure & function Enzymes are proteins that act as catalysts and control chemical reactions

What is DNA? a. DNA is often called the blueprint of life. b. In simple terms, DNA contains the instructions for making proteins within the cell. c. DNA stands for DeoxyriboNucleic Acid. Why is DNA called the blueprint of life?

I. Structure of DNA DNA is a polymer with repeating monomers called nucleotides Nucleotides have 3 parts: 1. Phosphate 2. Sugar (deoxyribose) 3. Nitrogen(ous) base

Nucleotide There are 4 Nitrogen(ous) Bases: 1. Adenine (A) Phosphate Deoxyribose Sugar There are 4 Nitrogen(ous) Bases: 1. Adenine (A) 2. Thymine (T) 3. Guanine (G) 4. Cytosine (C)

Watson & Crick- 1953 Proposed that DNA is… 1. Made of 2 strands of nucleotides 2. Complementary nitrogen base pairs were held together by weak hydrogen bonds. **The idea of the double helix shape was aided through the work of Rosalind Franklin. Franklin was taking X-rays of DNA, and one picture showed an “X” shape, a characteristic of a helix. Watson and Crick “borrowed” from Franklin, got their worked published, and went on to earn the Nobel Prize for their work. Franklin received no recognition.

Nitrogen(ous) bases pair up 1. Adenine pairs with Thymine A T 2. Guanine pairs with Cytosine G C

The amount of G = the amount of C Chargaff’s Rule: **In DNA: The amount of A = the amount of T The amount of G = the amount of C For example, If a DNA strand has 20% Guanine, how much Thymine does it have? 20% G = 20% C 20 + 20 = 40 100 – 40 = 60 30% A = 30% T

Hydrogen Bonds The nitrogen(ous) bases attract each other through 2. The two strands of DNA can be pulled apart because hydrogen bonds are weak.

DNA strands run in opposite directions = Basic DNA Structure P P T A S S P P C G S S Notice: DNA strands run in opposite directions = ANTI-PARALLEL

Importance of Nitrogen(ous) Base Sequence 1. Phosphate + Sugar = Backbone 2. Sequence of Nitrogen(ous) bases = storage of genetic information **Differences in sequences = differences in organisms

DNA Replication

Replication Facts a. DNA must be copied before a cell divides in a process called DNA replication

Replication Facts b. DNA is copied semi-conservatively (one old strand predicts base order of new strand). Original DNA DNA Template New DNA

Video – DNA Replication Replication Facts c. DNA is copied during the S or synthesis phase of interphase d. New cells will need identical DNA strands Video – DNA Replication

How DNA Replicates DNA is double stranded. An enzyme called helicase breaks the hydrogen bonds in between 2 strands. Replication Fork Parental DNA Molecule 3’ 5’ Helicase

Direction of Replication How DNA Replicates An enzyme called DNA polymerase adds on nucleotides to new strand based on parental DNA strand. DNA Polymerase Nucleotide 5’ 3’ Direction of Replication

How DNA Replicates An enzyme called DNA ligase acts as glue and pieces together nucleotides (phosphate bond) on new strand.

How DNA Replicates 4. The two DNA strands are anti-parallel (run in opposite directions). P O 1 2 3 4 5 G C T A

Parental Strand/ DNA Template DNA unzips Each original strand is used as a template for making new complementary base pairs.

Proofreading New DNA DNA polymerase initially makes about 1 in 10,000 base pairing errors. Enzymes proofread and correct these mistakes. The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors.

DNA Damage & Repair DNA in our body cells can be damaged by chemicals & ultraviolet radiation. Excision repair occurs when any of the 50 repair enzymes remove damaged parts of DNA. DNA polymerase and DNA ligase replace and bond the new nucleotides together.

DNA Replication Questions: What would be the complementary DNA strand for the following DNA template sequences? a. DNA template 5’-CGCTATGCA-3’ Complementary DNA _______________ 3’-GCGATACGT-5’

DNA Replication Questions: What would be the complementary DNA strand for the following DNA template sequences? b. DNA template 5’-TTACGACCT-3’ Complementary DNA _______________ 3’-AATGCTGGA-5’

DNA Replication Questions: What would be the complementary DNA strand for the following DNA template sequences? c. DNA template 5’-GCTACCGTA-3’ Complementary DNA _______________ 3’-CGATGGCAT-5’

Identify the parts of a nucleotide Where are the nitrogenous bases located? What is the shape of the DNA?

What is happening in the picture? What is the enzyme called that is responsible for this? What happens next to the separated strands?

Where do we get the building blocks (nucleotides) in order to build the new DNA strands? How do each of the nucleotides “know” their location?

What is happening in the picture? What is the enzyme called that is responsible for this?

What is the final result of the process? When in the cell cycle does this happen? For what purpose does this serve?