DNAWhat is it?

Why is it important?

What is DNA?

• deoxyribonucleic acid

• hereditary material

• Nearly every cell in a person’s body has the same DNA.

• Most DNA is located in the cell nucleus, but a small amount of DNA can also be found in the mitochondria.

• Made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T)

• The order, or sequence, of these bases determines the information available for building and maintaining an organism.

Check it! – answer the questions, get a stamp

• What does DNA stand for?

• What does it contain?

• How does it give us the information that we need?

Base Pairing• A with T: the purine adenine (A)

always pairs with the pyrimidine thymine (T)

• C with G: the pyrimidine cytosine (C) always pairs with the purine guanine (G)

• only with A & T and with C & G are there opportunities to establish hydrogen bonds

• DNA bases pair up with each other, A with T and C with G, to form units called base pairs.

• also attached to a sugar molecule and a phosphate molecule

• Base pair + sugar molecule + phosphate molecule = nucleotide

• Nucleotides are arranged in two long strands that form a spiral called a double helix.

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• What is a base pair?

• What are the base pairs of DNA? What is made between the base pairs?

• Describe a nucleotide.

• it can replicate• Each strand of DNA in the double helix can serve as

a pattern for duplicating the sequence of bases.• This is critical when cells divide because each new

cell needs to have an exact copy of the DNA present in the old cell.

• Genes, which are made up of DNA, act as instructions to make molecules called proteins.

• Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.

The C+G:A+T ratio varies from organism to organism (particularly among the bacteria), but within the limits of experimental error A = T and C = G Relative Proportions (%) of Bases in DNA

Can you figure out proportions of the missing bases?

Organism A T G C

Human 30% 19%

Chicken 29% 21%

Wheat 27% 27%

Yeast 18% 18%

E.coli 24% 26%

DNA Replication• Prior to cell division, the DNA material in the

original cell must be duplicated so that after cell division, each new cell contains the full amount of DNA material.

• The process of DNA duplication is usually called replication.

• The replication is termed semi conservative since each new cell contains one strand of original DNA and one newly synthesized strand of DNA.

• The original strand of DNA serves as a template to guide the synthesis of the new complementary DNA.

How does replication happen?• Several enzymes and

proteins are involved with the replication of DNA.

• At a specific point, the double helix of DNA is caused to unwind using the enzyme helicase.

• Proteins are available to hold the unwound DNA strands in position.

• Each strand of DNA then serves as a template to guide the synthesis of its complementary strand of DNA.

• DNA polymerase DNA polymerase is used to join the nucleotide units together.

CHECK IT!

1. What enzyme aids in unzipping the DNA?2. What is the job of DNA polymerase?3. Why is DNA replication considered Semi-

conservative?

DNA vs. RNA• Differs from DNA

– Single Stranded– Ribose Sugar– Base Pairs A-U, G-C– Are there T’s in RNA?– A, U no T’s in RNA

• RNA assists DNA in manufacturing needed proteins

• Uracil is found only in RNA.

RNA - Transcription• proceeds in much the

same fashion as the replication of DNA and also follows the base pairing principle

• Again, a section of DNA double helix is uncoiled and only one of the DNA strands serves as a template for RNA polymerase enzyme

RNA - Transcription• One major difference is

that adenine, on DNA codes for uracil in RNA rather than thymine as in DNA.

• Remember that thymine is not found in RNA

• Thymine in DNA still codes for adenine on RNA not uracil, while the adenine on DNA codes for uracil in RNA.

Check it• Name three differences between RNA and DNA in

the transcription process.

• What is the purpose of RNA polymerase?

• In transcription A pairs with ____T pairs with ____G pairs with _____C pairs with _____

RNA Translation: aka Protein Synthesis

• The problem: How does a particular sequence of nucleotides specify a particular sequence of amino acids?

• By means of transfer RNA molecules, each specific for one amino acid and for a particular triplet of nucleotides in mRNA called a codon.

STEP 1:

• The first step in protein synthesis is the transcription of mRNA from a DNA gene in the nucleus.

• The RNAs migrate from the nucleus into the cytoplasm.

• Prior to the beginning of the protein synthesis, all of the component parts are assembled in the ribosome which is the brown/tan structure in the graphic.

STEP 2: Initiation:

• In the cytoplasm, protein synthesis is actually initiated by the AUG codon on mRNA.

• The AUG codon signals both the interaction of the ribosome with m-RNA and also the tRNA with the anticodons (UAC).

STEP 3: Elongation:

• tRNA’s bring in the anticodon, which has a specific amino acid attached to it.

• As the anticodons match up with the codons, the amino acids join forming a chain (sequence).

• This sequence codes for a specific protein, which becomes a characteristic within the organism.

Step 4: Elongation and Termination:

• When the stop signal on mRNA is reached, the protein synthesis is terminated.

• There are three stop codons: UAA, UAG and UGA

• The peptide (amino acid) chain leaves the ribosome and gets packaged by the Golgi Body and turned into trait.

• The ribosome is now ready to repeat the synthesis several more times.