DNA(DEOXYRIBONUCLEIC ACID)

The Search for Genetic Material Up to this point, we know that there are

chemical factors called genes that pass traits from one generation to the next

What was unknown though was what were genes made of

Scientists begin to search for the genetic material

First Discovery

Fredrick Griffith was a scientist that was interested in bacteria Specifically what/how bacteria caused disease such as

pneumonia

He isolated 2 similar types of pneumonia bacteria from mice and grew them in culture Smooth Bacteria – caused pneumonia Rough Bacteria – didn’t cause pneumonia

When Griffith injected the mice with the 2 strains, he saw very different results Smooth Strain - Mice die Rough Strain – Mice live

Griffith wanted to know if the disease causing strain was the result of a poison produced by the bacteria

More Experimenting

To figure out what exactly was causing the mice to die, he heat killed the disease causing (smooth) bacteria Mice injected with heat killed smooth bacteria, the

mice live Since the mice are still alive, the initial death was

not caused by a poison

Griffith then combines heat killed smooth bacteria and live rough bacteria Mice injected with this combiniation results in the

mouse death

When Griffith removed the bacteria from the dead mice, he saw that they were all filled with the smooth (disease-causing) bacteria

How did this happen?

Transforming Principal

Somehow the heat killed strain passed their disease causing abilities to the harmless strain

Griffith called this process transformation One strain (harmless rough) had been

permanently changed into another (a disease causing strain)

The information that caused disease must have transformed the information originally that did not cause disease Since the ability to cause disease was inherited, it

must be caused by a gene

Griffith’s Transforming Experiment

The Search Continues…

In 1944 Oswald Avery repeated Griffith’s work and tried to determine what the genetic material was

He used the same bacteria (S &R) and injected them with different enzymes1. First injected with enzymes to destroy molecules such as

proteins, carbohydrates, lipids and RNA Transformation still occurs

2. Second, he injects with enzymes to destroy DNA Transformation does not occur

This is the first ground-breaking evidence that shows that DNA is the genetic material

Avery presented this material in 1944 but there was still skepticism…

Still wanna know…

Alfred Hershey and Martha Chase studied bacteriophages (phage)

viruses that infect bacteria

They phages are made up of a DNA core and protein coat

How does a virus work? They land on a cell and inject their genetic

material into the cell

DNA it is!

They placed radioactive markers for 2 different materials on phages and injected them into bacteria1. First was a sulfur marker

If protein was the genetic material injected, there would have been radioactivity inside the bacterium

No radioactivity was seen

2. Second was a phosphorus marker If DNA was the genetic material injected, there

would have been radioactivity inside the bacterium Radioactivity was detected!

Finally proves that DNA is the genetic material

Hershey-Chase Experiment

DNA structure

Scientists were happy to know that DNA was the genetic material

They now had new questions1. How did genes (and therefore DNA) carry genetic

information from one generation to the next2. How was it that DNA created the characteristics of

organisms3. How was it that DNA was copied with each cell

division

They knew DNA had to be a very special molecule

DNA Structure

DNA is a long molecule made of units called nucleotides

Each nucleotide is made of 3 parts1. 5 Carbon Sugar – Deoxyribose2. Phosphate Group3. Nitrogenous Base (nitrogen containing)

There are 4 different nucleotides that make up DNA and the only difference is the nitrogenous base

Forms the backbone

Forms the steps to the ladder

Nitrogenous Bases

There are 2 groups of nitrogenous bases

1. Purines (have 2 rings)1. Adenine (A)2. Guanine (G)

2. Pyrimidines (have 1 ring)1. Cytosine (C)2. Thymine (T)

Nitrogenous Bases

Thymine

Cytosine

How does DNA do it?

Scientists were baffled to think that 4 different nucleotides were responsible for all of a genes properties

They wondered how these 4 molecules could be strung together to make so many different genetic possibilities

Chargaff’s Rules

In 1950 Erwin Chargaff looked at the relationships of the 4 nitrogenous bases in samples of DNA

He saw that percentages of Adenine (A) were always the same as Thymine (T)

He saw that percentages of Cytosine (C) were always the same as Guanine (G)

DNA Model

In the early 1950’s Rosalind Franklin used x-rays to try to determine the structure of DNA.

These x-rays showed an x pattern with a circle around it This image suggested that there

were 2 strands with the nitrogenous bases located near the center

DNA Model

Franklin’s coworker, Maurice Wilkins showed the data without Franklin’s consent to 2 scientists James Watson and Francis Crick

Watson and Crick were trying to understand and determine the structure of DNA by building a 3-D model They use Franklin’s finding to support their own data

and determine the structure of DNA - The Double Helix

They present this double helix in a published paper in 1953

DNA Model

Watson, Crick and Wilkins were awarded the Nobel Prize in 1962 for their discovery

DNA Model Continued…

As mentioned before, there are 4 nucleotides A, T, C and G

It was discovered that hydrogen bonds form between the different bases

Remember Chargaff’s discovery A=T and C=G

Principal of Base Pairing - A always binds to T and C always bind to G The pairs bind in this manner due to their size and

hydrogen bonding.

The DNA Molecule

DNA Replication

The process of copying DNA within a cell Occurs within the nucleus of the cell 3 steps

1. DNA Helicase breaks hydrogen bonds between base pairs

- Replication Fork- the point at which 2 chains separate

2. Each chain serves as a template for a new nucleotide chain- DNA polymerase binds to the separated chains and adds corresponding base pairs in available spots

3. 2 identical molecules of DNA are formed - Semi-conservative process (1 original and 1 new strand

of DNA results)

DNA Replication Continued… This is a process that occurs

spontaneously and at many point (makes it very fast!)

When the replication is complete, the cell is ready to undergo cell division

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RNA (ribonucleic acid)

DNA is the mother copy of the recipe for making proteins.

DNA must be copied and edited to make proteins.

How does this happen? With the production of RNA (transcription) and

then RNA’s production of protein (translation). Structure

5 carbon sugar (ribose) Phosphate group Nitrogenous Base (A-adenine, U-uracil, G-

Guanine, C-cytosine)

Transcription

The process of copying a DNA into RNA 3 steps

1. RNA polymerase recognize a start site on the DNA causing the DNA to unravel

2. The RNA polymerase uses one of the strands as a template and forms complementary base pairs (C pairs with G and A pairs with U –Uracil)

3. Once transcription is complete, the new RNA strand separates from the DNA and the DNA helix zips together again

Transcription Continued…

Produces 3 different types of RNA1. Messenger RNA (mRNA)

- carries copies of instructions for copying amino acids into proteins

- Serves as “messengers” from DNA to rest of the cell

2. Ribosomal RNA (rRNA)- Proteins are assembles here (make up the

ribosome)

3. Transfer RNA (tRNA)- brings amino acids from the cytoplasm to a ribosome to help create a protein

http://www.youtube.com/watch?v=NJxobgkPEAo

Translation

The process that converts, or translates, mRNA into proteins – Protein Synthesis

The language of nucleic acids Codons- the 3 letter sequence of

nucleotides or “words” that code for a particular amino acid. Start codon – starts translation (AUG –

methionine) Stop codon – stops translation (UAA, UAG

and UGA)

Translation Process

The mRNA is carried out of the nucleus and into the cytoplasm to the rRNA which is located in the ribosome

The codons of the mRNA match to the complimentary anti-codon of tRNA which also is carrying an amino acid

As more and more tRNA’s are matching up with the corresponding mRNA’s, more amino acids are coming into contact

The ribosome breaks the bond between the amino acids and the tRNA and links the amino acids together

This process of creating proteins is known as Protein Synthesis

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Mutations

A change in DNA that affects the genetic information.

There is a series of proofreading and repair processes carried out by enzymes

What causes mutations? Errors in replication Mutagens – chemicals, radiation (UV, X-ray)

Types of Mutations

Gene mutations2 types1. Point mutations – 1 nucleotide is substituted for

another (changes the amino acid)

2. Frameshift mutations – involves the insertion or deletion of a nucleotide (changes the amino acid sequence)

Ex) THE CAT ATE THE RAT (if you remove the first E…) THC ATA TET HER AT (the letters now don’t make

sense)

Mutations Continued…

Chromosomal mutations Occurs during meiosis when chromosomes

don’t exchange DNA properly Causes 2 situations called gene duplication

or translocation Impact

Depending on the mutation, it could cause a change in phenotype of the individual or in the offspring