The Discovery of DNA as the genetic material

Frederick Griffith

Oswald Avery

Hershey and Chase

Franklin

Watson and Crick

Chargaff

Write the term and definition

• 1) Homeostasis• 2) Prokaryotic• 3) Anaerobic Respiration• 4) Mitosis• 5) Replication

DNA and its Functions

DNA Structure• DNA is described as a double helix which

means two strands of nucleotides are attached together and then twisted.

• In order to put nucleotides together into a strand to make one side of DNA the phosphate of one is attached to the sugar of the next creating a backbone.

• Because the phosphates and sugars create the backbone, the bases stick out from this backbone and will be bound to the bases sticking out from the other strand to connect the two together using weak hydrogen bonds.

• There are four bases of DNA (adenine, thymine, cytosine, and guanine). The A base always connects to a T base and C always connects to a G base.

• This means:

Structure Allows Functions• Functions of genetic info:

1) Easily copied for each new cell created.2) Must be passed to next generation.3) Provide instructions of how to make proteins which build structures and help

carry out chemical reactions.• Review:

- All living things are based on the same genetic code. The sequence of the code is different in every organism creating the variations of life on the planet.- Only small sections of DNA are expressed at a time. Most cells of an organism have the same DNA but don’t perform the same function which means they do not express the same sections. Cell specialization.

Function 1: Easily copied for each new cell created

• Process to achieve this function: Replication

• Purpose of process: to make an exact copy of the existing DNA strand so that each new cell possesses the same genetic info after division.

• Structures that help this process occur:– Two sides of DNA are connected

by weak hydrogen bonds which are easy to separate.

– Bases always pair the same A—T and C—G.

Function 1: Easily copied for each new cell created• Process of Replication

– An enzyme binds to DNA and breaks the hydrogen bonds holding the two sides together.

– Another enzyme brings nucleotides that are currently not being used to the DNA strand.

– The nucleotides match the bases on each side of the existing DNA strand (template) with A matching T and C matching G.

– When complete each new DNA strand is made of one side that is the original DNA and one side that is a newly constructed strand. This is why the process is called semi conservative.

Function 2: Must be passed to next generation.

• Replication occurs during S phase of the cell cycle.

• Then the cell divides through the process of mitosis or meiosis passing the genetic information on to the next generation.

Function 3: Provide instructions to make proteins Step 1: Transcription• Review:

– DNA must stay in the nucleus.– The sequence of bases in DNA provides to

coded instructions to assemble proteins.– Proteins are created in ribosomes.

• Problem – The instructions on how to make proteins

(DNA) are in the nucleus and the proteins are made in the ribosomes (not in the nucleus). The instructions must be taken to the ribosomes somehow.

• Solution: Step 1 Transcription– DNA is copied into a similar strand called

RNA. This allows many “photocopies” of the instructions to be made and taken out of the nucleus.

• Structures that help this process occur:– Two sides of DNA are connected

by weak hydrogen bonds which are easy to separate.

– Bases always pair the same A—T and C—G .

• RNA differences from DNA– RNAs are only one strand

because they are only matched to one side of DNA and then separated.

– The sugar used in the nucleotides of RNA is ribose instead of deoxyribose.

– The four bases for RNA are CGA and uracil instead of CGA and thymine.

Step 1: Transcription

Step 1: Transcription• Purpose: to make RNA copy of the

gene (section of DNA that provides instructions to make a protein) so that the instructions can leave the nucleus.

• Location: in the nucleus.• Process:

– An enzyme binds to DNA and breaks the hydrogen bonds separating the two sides.

– RNA nucleotides are MATCHED to one side of DNA (remember if there is an A in the DNA the RNA match is a U).

– Once complete the RNA molecule is released and leaves the nucleus. The DNA molecule coils back together.

• Types of RNA made: All three types of RNA are used in protein production, but they each have a specific function.– Messenger RNA (mRNA): a RNA copy of the DNA

gene (section of DNA that codes for a protein). This type of RNA will provide the instructions of how to construct the protein.

– Transfer RNA (tRNA): this type of RNA is coiled back on itself leaving three bases exposed at one end that will MATCH the mRNA bases. At the other end a specific amino acid is attached. Each tRNA carries one specific amino acid to the ribosome so that they can be assembled together.

– Ribosomal RNA (rRNA): this RNA will attach to proteins that have already been created to create the ribosome, the site of protein production.

Step 1: Transcription

Write and Answer the Questions• Replicate CTTATGCGAA GAATACGCTT• Transcribe CTTATGCGAA GAAUACGCUU• What is made during replication? Exact copy of DNA• Why does replication occur? Every time the cell divides

(asexual, growth, repair) each new cell needs its own copy of DNA.

• What is made during transcription? RNA • Why does transcription occur? It make an RNA copy that can

leave the nucleus to go to the ribosome to provide instructions to make proteins

• Where in the cell does replication occur? nucleus• Where in the cell does transcription occur? nucleus

• Problem: The four bases of RNA have to be arranged and code for 20 different amino acids.

• Solution: read the bases three at a time creating 64 different codes (4x4x4=64) which is enough to code for the amino acids. This means that some amino acids will have more than one code.

Step 2: Translation

Step 2: Translation• Purpose: use the instructions

from mRNA to assemble amino acids together in the correct order to make a protein.

• Location: ribosome• Process:

– The two pieces of the ribosome bind to the mRNA at the start codon AUG (a codon is the three base sequence on mRNA that codes for an amino acid).

– The tRNA that is responsible for carrying the amino acid that AUG codes for will bring that amino acid to the ribosome.

– Once in the ribosome the anticodon of the tRNA (the three bases that MATCH the codon) will bind with the codon so that the structure can stay connected for a little while.

– Then the next codon is read and the tRNA that MATCHES that codon brings the amino acid to the ribosome.

– Since ribosomes are small only two tRNAs can fit at a time.

– The amino acid that was on the first tRNA will attach to the amino acid on the second tRNA using a peptide bond. This is the beginning of the amino acid chain that will eventually fold and become a protein.

– The first tRNA leaves and the process is repeated until a stop codon is reached. Then all elements are released and the long chain of amino acids is folded into a functioning protein.

Step 2: Translation

Problems with the Processes• Mutations

– Addition: adding an extra base to the DNA chain being created.

– Deletion: leaving out a base when creating the DNA chain.

– Substitutions: Inserting the wrong base in the DNA chain being created.

• Caused by radiation, chemical exposure, or random/spontaneous mistakes in process.

• Mutations in the DNA sequence can cause changes in the amino acid sequence resulting in a change in protein structure and function.

• Only mutations that occur in sex cells can be passed on to offspring.