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DNA and RNA
Bio 100
Tri-County Technical College
Pendleton, S. C.
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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
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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
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A Chromosome
Gene 1 Gene 2 Gene 3 Gene 4 Gene 5
Protein 1 Protein 2 Protein 3 Protein 4 Protein 5
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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
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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
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DNA base-pairing
• Adenine pairs with thymine
• Guanine pairs with cytosine
Adenine Guanine Thymine Cytosine Adenine
Thymine Cytosine Adenine Guanine Thymine
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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.
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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
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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
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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.
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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
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Semiconservative??
• neither original strand is discarded
• each original strans serves as a template for a new strand
• half is new, half is original
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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
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Protein Synthesis
Gene (DNA)
messenger-RNA
Protein
Transcription (takes place in the nucleus)
Translation (takes place in the cytoplasm)
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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.
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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”
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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
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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
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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.
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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.
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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
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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.
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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
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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
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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?
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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.
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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
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