DNA and RNA

Bio 100

Tri-County Technical College

Pendleton, S. C.

Here a little..there a little

• Griffith’s Transforming Factor Experiment

• Hershey and Chase and bacteriophages

• Bacteriophage (phage) is a virus that infects bacteria

• Watson and Crick and the world has never been nor will it ever be the same again

• Rosalind Franklin provided the key for Watson and Crick’s success

Genes and DNA

• Nucleus of a cell contains chromosomes

• Chromosomes are made genes

• Genes are made of DNA.

• Each gene is a part of a chromosome that codes for a protein or a portion of a protein.

• The Human Genome Project

A Chromosome

Gene 1 Gene 2 Gene 3 Gene 4 Gene 5

Protein 1 Protein 2 Protein 3 Protein 4 Protein 5

DNA composition• Composed of nucleotides

• Nucleotide– base, sugar, phosphate

• Sugar of DNA: deoxyribose

• Bases of DNA– adenine, thymine, guanine, cytosine

• Sugar is bonded to the base and the phosphate

How are nucleotides put together?

• Sugars and phosphates are linked to each other to make a strand of nucleotides with the bases sticking out from the strand.

• DNA has two of these strands.

• The whole thing looks like a ladder.– Sides are made of sugar-phosphates– Rungs are made of bases bonded with hydrogen

bonds

DNA base-pairing

• Adenine pairs with thymine

• Guanine pairs with cytosine

Adenine Guanine Thymine Cytosine Adenine

Thymine Cytosine Adenine Guanine Thymine

RNA• Composed of nucleotides

• Sugar of RNA -- ribose

• Bases of RNA– adenine, uracil, guanine, cytosins

• RNA is single stranded

• Sugars and phosphates are linked to each other to make a strand of nucleotides with the bases sticking out from the strand.

Kinds of RNA

• Messenger-RNA (m-RNA)– produced in the nucleus from DNA

• Transfer-RNA (t-RNA)– found in the cytoplasm attached to amino acids

• Ribosomal-RNA (r-RNA)– one of the components of ribosomes– ribosomes also contain protein

Physiology of DNA

• What does DNA do?

• DNA is replicated.– an exact copy is made– each daughter cell resulting from cell division

has an exact copy of all the genes

• DNA is involved in protein synthesis.– contains the blueprint for all the proteins the

cell needs to produce

DNA Replication

• 1. An enzyme breaks the attachments between the two strands of DNA.

• 2. The two strands unzip exposing the bases.• 3. Complementary nucleotides pair up with the

bases.• 4. DNA-polymerase synthesizes covalent bonds

between sugars and phosphates of adjacent nucleotides.

Base-pairing

• adenine-containing nucleotide pairs with a thymine-containing nucleotide

• guanine-containing nucleotide pairs with a cytosine-containing nucleotide

• The result is an accurate duplicate of the originial DNA strand.

• The process is called semi-conservative DNA replication

Semiconservative??

• neither original strand is discarded

• each original strans serves as a template for a new strand

• half is new, half is original

DNA Replication - When

• During the life cycle of the cell.

• Just before cell division

• During a stage known as the “synthesis” stage

• Chromosome number is preserved

Protein Synthesis

Gene (DNA)

messenger-RNA

Protein

Transcription (takes place in the nucleus)

Translation (takes place in the cytoplasm)

Transcription

• DNA contains the code for the position of each amino acid in a protein.

• Three-letter code

• Transcription transfers this code from DNA to messenger-RNA

• Transcription takes place because of base-pairing relationships.

Base-paring -- transcription

• During transcription– adenine on DNA pairs with uracil on RNA– there is no thymine in RNA– guanine pairs with cytosine

• Code is transferred to m-RNA

• A sequence of three bases on m-RNA that corresponds to the DNA code is called a “codon”

What happens next?

• m-RNA leaves the nucleus and enters the cytoplasm where it attaches to a ribosome.

• t-RNA brings amino acids to the ribosome attached to the m-RNA

More about t-RNA

• has two ends

• one end is attached to the amino acid

• the other end contains an anticodon which is complementary to a specific codon on the m-RNA

• when the t-RNA approaches the m-RNA the anticodon matches with its complementary codon

Codon-anticodon Matching

• Insures that the correct amino acid is placed where it belongs in the sequence of amino acids

• As the codon-anticodon matching continues amino acids are placed beside each other in the sequence dictated by the original code on the DNA on the chromosome in the nucleus.

Role of the ribosome

• The ribosome moves along the m-RNA strand and wherever there are two adjacent amino acids the ribosome synthesizes a peptide bond between them.

• The result is chain of amino acids that eventually becomes a part of a large protein.

Let’s do transcription --

Write the m-RNA strand that will be produced from the DNA strand below.AAA AGT GGT TGG TTT CTA CCC CCC CAA

The Rules:

adenine on DNA pairs with uracil on m-RNA

thymine on DNA pairs with adenine on m-RNA

guanine on DNA pairs with cytosine on m-RNA

Let’s Do Translation

• 1. Write down the m-RNA you created from the DNA in the previous slide.

• 2. Look up the amino acids which are coded for by the codons on the m-RNA stand.

• When asked to do this on a test, the codon chart will be provided.

Review roles of RNA

• m-RNA– brings the code from the nucleus in the form of

codons to the ribosomes in the cytoplasm

• t-Rna– brings the amino acids to the ribosome where

proteins are being synthesized– contains the anticodon which matches with the

codon

What Happens With Mutations?

• A mutation is a change in the content of the genetic message.– A change in the base sequence of one or more

genes.

• There are different kinds of mutations.

• Point mutation– involves only one or two base pairs in the DNA

Mutation Illustration

AAA AGT GGT TGG TTT CTA CCC CCC CAA

The above is the DNA strand we were working with. Let’s say a mutation occurred to alter this strand as shown below.

AAA AGT GGT TGC TTT CTA CCC CCC CAA

Transcribe this into m-RNA and then translate the m-RNA into a protein. What are the results?

Sickle-Cell Anemia

• A mutation occurred sometime in the past which caused the gene for hemoglobin in affected individuals to translate into defective hemoglobin r/t normal hemoglobin.

• The red cells are sickle shaped in these individuals and do not carry hemoglobin well.

Moving right on along

• Lytic viral replication cycle

• Lysogenic viral replication cycle

• Retroviruses, reverse transcriptase, and HIV

• Just the tip of the iceberg…it really wasn’t the ice above the water than sank the Titanic

• Emerging viruses (SARS is latest example)

• Most authorities agree that it is only going to get WORSE