Notes: DNA

I. What is DNA?• DNA (deoxyribonucleic acid) is a type of

molecule found in the nucleus of all cells that contains instructions for the cell and determines the traits of the organism.

• Important: All the somatic (body) cells in an organism have the same exact DNA.

• Do you think every cell uses every gene? _____ No.

But, the code is identical in every cell of your body because all of these cells divided by mitosis from the same cell (the first cell formed by your mom’s sperm and your dad’s egg).

II. How was DNA discovered?

• Rosalind Franklin did experiments with X-rays to determine the structure of DNA.

• Watson and Crick then completed the model of the DNA molecule.

II. How was DNA discovered?

• We now know that DNA has a “double helix” structure (like a windy stair case).

II. How was DNA discovered?

Watson and Crick:

III. What is the structure of DNA?

DNA is made of building blocks called nucleotides.

Each nucleotide has 3 parts:

Phosphate group

Deoxyribose (sugar)

Nitrogenous base

Draw a nucleotide:

Deoxyribose sugar

Which part of the nucleotide contains the genetic instructions? The phosphate, the sugar, or the nitrogenous base?

_______________________________

The nitrogenous base (A, T, G, or C) –the order of these determines the traits of the organism

Put these in order from smallest to largest: Gene, nucleotide, chromosome

Nucleotide Gene Chromosome

Nucleotide: a small piece of DNA

Gene: a small piece of the chromosome

Chromosome: an entire strand of DNA

How do you know which bases go with which?

• There are 4 base pairs in DNA– Adenine– Thymine– Guanine– Cytosine

Complimentary Bases

• Only certain bases can “pair up” along two strands of DNA.

AT

GC“Austin Texas, Go Cougars”

• The two strands are called complimentary.

DNA Structure

• Each strand of DNA has a 5’ end and a 3’ end. (This has to do with the arrangement of the molecules)

• The two strands are called antiparallel because the two strands are facing opposite directions.

Practice: Write the complimentary strand:

5’ TTGCATTCA 3’

_________________3’ AACGTAAGT 5’

Question: If 20 % of a chromosome is made up of guanine, what percent of the nitrogenous bases are cytosine? ___________

Why? ___________________________ ________________________________

20%

Because every C is paired with a G. The same is true for A and T

How does DNA get replicated (copied)?

• Each time a cell divides by mitosis, the DNA in that cell must be replicated (copied).

• This insures that both daughter cells have a complete set of genetic material.

DNA

• Step 1: The DNA is unzipped at the base pairs by a protein called helicase.

• Step 2: Each nucleotide on the parent strands are paired with a new complimentary nucleotide. (A-T, G-C)

• Step 3: Nucleotides are bonded together by a protein called polymerase.

• Step 4: Two DNA molecules are formed - each with a parental strand and a new strand.

How does DNA get replicated (copied)?

• This process is called semiconservative because the new strand is ½ old DNA and ½ new DNA.

What does semiconservative mean?

Watch this animation of DNA Replication:http://www.youtube.com/watch?v=teV62zrm2P0

Write this song down to help you remember the parts of DNA: (to the tune of row row row your boat)

We love DNA

Made of nucleotides

Sugar, phosphate, and a base

Going down the sides