Unit: Molecular Genetics

The traits that you have are mainly due to the genesthat you have inherited from your parents.

DNA (deoxyribonucleic acid) - nucleic acid found in allcells that controls cellular activity

DNA is organized into genes. Humans have about30,000 genes (made of DNA)!

Gene - a sequence of DNA that codes for a protein andthus determines a trait.

• At specific times during the life of an individual, cells and organisms use the information that is stored in our DNA. (in other words, the gene is expressed resulting in the production of a protein)

• Genes provide a set of instructions, or a genetic program for an individual’s development.

 

Griffith and Avery (1928, 1944 )

• Their work yielded results that suggested that the nucleic acid DNA stores and transmits genetic information from one generation to the next.

Hershey and Chase (1952)

• Their work showed that viral genes are made of DNA, not protein.

 

 • Erwin Chargaff was a biochemist working at

Columbia University in New York.

• In 1949, he made the observation that the amount of adenine (A) was always equal to the amount of thymine (T) for each organism that he studied. The amount of Guanine (G)was also always equal to the amount of Cytosine (C).

 1950’s: X-Ray diffraction was used to study thestructures of molecules.  • How it works: X rays bounce off of the object and are

scattered in a pattern onto a piece of film. Scientists then shine a light on the object and study its shadow.

 • Rosalind Franklin and Maurice Wilkins used this

technique the winter of 1952 and looked at diffraction photos of fibers of DNA.

  

The X pattern observed ischaracteristic of ahelix shape.

Used the data collectedby Franklin, Wilkins, Chargaff and others topropose the 3D doublehelix structure for DNA

Their proposal wassuch an outstandingcontribution to sciencethat they received aNobel Prize in 1962

Take 2 minutes to summarize what you have

Learned by answering the following question:

How was the function and structure of DNA

determined?

DNA molecule is a long, twisted, double-stranded structure

( like a spiral staircase)

  Each strand

consists of a chain of smaller units, called nucleotides

1. A five carbon sugar known as deoxyribose

 2. A phosphate group 

3. A nitrogen base: This is the part of DNA that can have alternate forms. There are 4 different nitrogen bases

• A: Adenine• T: Thymine• C: Cytosine• G: Guanine

Nucleotides are connectedto other nucleotides viacovalent bonds, whichconnect the sugar from onenucleotide to the phosphategroup of another.

• They can be connected in any order.

Adenine (A) & Guanine (G) are classified asPurines.

 Purines are largermolecules. They are madeof two rings of carbon andnitrogen atoms.  

Cytosine (C) & Thymine (T) are classified asPyrimidines

Pyrimidines are made of a single ring of carbonAnd nitrogen atoms.

5-Carbon Sugar andPhosphate group  The sugars/

phosphate form the backbone of each strand

They are like the side rails of a ladder

 This is what connects the two strands, like the rungs of a ladder

A base from one strand pairs with a base from the other strand. The two bases face each other

Answer: WeakHydrogen bonds that form betweenthe pairs of bases.

It is important thatthese bonds break’Easily because DNAneeds to ‘unzip’ inorder to replicatebefore cell division!

Adenine pairs onlyThymine (A – T)

Guanine pairs onlywith Cytosine (G – C)

 

Notice: A Purine onone strand of DNAalways pairs with a pyrimadine on theother strand.

• The structure and size of the nitrogen bases allow for this pairing arrangement only.

Base-pairing rules – the pairingarrangement of thenitrogen basesbetween the 2 strands

• This is supported by Chargaff’s observations.

The two strands of DNA that make up thedouble helix are complementary to each

other.

This means that the sequence of one strand

determines the sequence of bases on the other

strand.  

Sample sequence of DNA on one strand:

TCGAACT  What is the sequence of DNA on the otherstrand?

Answer: AGCTTGA

Take two minutes to summarize what you havelearned about the structure of DNA. Be sure to

include the following in your summary

Describe the SHAPE of the DNA molecule. Name the building block of DNA, along with itssubcomponents. Describe how the molecule is

held together.

http://video.nationalgeographic.com/video/salmon_sockeye