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Unit 6: DNA & Protein SynthesisCh. 9: Chemistry of the Gene
Ch. 10: From Genes to ProteinsDNA = Deoxyribonucleic Acid
300
Video PPT:Lies, Thieves, & DNA
• How did each of the following scientists contribute to our knowledge of DNA and the genetic code?– Friedrich Miescher– Frederick Griffith– Oswald Avery, Colin MacLeod, & Maclyn McCarty– Alfred Hershey & Martha Chase– P.A. Levene– Erwin Chargaff– Rosalind Franklin & Maurice Wilkins– James Watson & Francis Crick
Describing DNA
• Who first described the true shape of DNA?– James Watson &
Francis Crick
• What shape did they use to describe DNA?– double helix
DNA is a Polymer…
• What is the monomer that repeats to make DNA?–nucleotide
What are the parts of a nucleotide?• phosphate group
• sugar (deoxyribose)
• nitrogenous base– 4 kinds:
• Adenine (A)• Thymine (T)• Guanine (G)• Cytosine (C)
– base gives name to nucleotide
adenine, thymine, guanine, cytosine
• 4 kinds of nitrogen bases:– Purines (2 rings)
• Adenine• Guanine
– Pyrimidines (1 ring)• Thymine• Cytosine
• Always pair– A – T
– G – C
What are the parts of a nucleotide?
A Model of DNA• Is DNA single or double stranded?
– double-stranded• 2 chains of nucleotides bonded together
– shape = “double helix” (spiral staircase)
DNA Structure Video
nucleotide
Purines Pyrimidines
1
2
3
4
A Model of DNA• What parts make up the strands?
– “backbone”/sides• alternating sugars (deoxyribose) & phosphates
– “rungs”/steps (attached to sugar)• complementary nitrogenous base pairs
– A - T– G - C
» held by hydrogen bond
DNA Structure Video
nucleotide
Purines Pyrimidines
1
2
3
4
• Strands are complementary (A-T & G-C) & antiparallel (opposite directions/upside down)
Where is DNA found in eukaryotic cells?
• DNA is contained in chromosomes (chromatin) within the nucleus.
sugar-phosphate “backbone” & nitrogenous base “rungs”
DNA Replication• What is another word for replication?
– copying
• Why must DNA replicate?– so when cell divides, each new
cell gets a copy of DNA
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/dna-rna2.swf
DNA Replication• When does DNA replicate?
– before the cell divides (before mitosis or meiosis)
• Where does DNA replicate?– inside the nucleus
• 1. “Helicase” enzyme unzips DNA molecule along H bonds (between bases) forming two “parent strands”.
What are the steps in DNA replication?
• 2. “Parent” strands act as templates…– “parent” strand’s base is paired up with
complementary “new” nucleotide with help of “DNA Polymerase” enzyme
• forming complementary daughter strand– ex. “G” on “parent” DNA polymerase brings “C”
What are the steps in DNA replication?
T
C
• 3. Sections of nucleotides (Okazaki Fragments) are joined by enzyme DNA ligase.– Now, have 2 exact copies of original DNA molecule.
• & when cell divides, each “new” daughter cell gets a copy
What are the steps in DNA replication?
DNA Replication Video
T
C
What are the steps in DNA replication?
Let’s Practice… Predict the next base…
Let’s Practice… Finish the replications…
Let’s Practice… Finish the replications…
Deoxyribonucleic Acid vs. Ribonucleic Acid
DNA RNA
• sugar = deoxyribose
• sugar = ribose
• double stranded
• single stranded
•CANNOT leave nucleus
•Can move between nucleus & cytoplasm
• N bases = adenine, thymine, guanine, cytosine
• N bases = adenine, uracil, guanine, cytosine
The DNA Code & Protein Synthesis• What is a gene?
– section of DNA that codes for synthesis of a specific protein
• Which part of DNA carries the code for a protein?– the nitrogen bases
• & their order– change sequence amino
acid (usually) changes & protein made (usually) changes
• So, the big question is… – How does the DNA code in a cell’s nucleus
get to the ribosomes where proteins are synthesized?• transcription & translation
The DNA Code & Protein Synthesis
The Central Dogma of Biology
Transcription• What is
transcription?– DNA code is
“transcribed”/copied into mRNA codons
• When does transcription happen?– when a protein is
needed
Transcription• Where does transcription happen?
– in the nucleus
• What does transcription make?– A single-stranded messenger RNA (mRNA)
• made from instructions/order of the bases in the DNA
Steps of Transcription• When a protein is needed:
– 1. DNA near needed gene gives signal (“start codon”)
– 2. DNA unzips (w/ help of RNA polymerase)
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf
Steps of Transcription• 3. RNA polymerase pairs complementary RNA
nucleotides with unzipped DNA strand– until reaches a “stop codon”
• 4. Nucleotides bond to form single RNA strand of mRNA (messenger RNA)
• 5. The 2 DNA strands rejoin & mRNA moves out of nucleus into cytoplasm
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf
mRNA Carries DNA’s Code Using “Codons”
• What is a codon?– 3 consecutive bases
on mRNA that codes for a particular amino acid• 64 possible combos
– Some amino acids have more than one codon.
mRNA Carries DNA’s Code Using “Codons”
• How can we figure out the amino acid coded for by a codon?– a codon
chartWhat amino acid does the AUG codon code for?
It must be in
this row
It must be in this
square
It must be in this
amino acid
mRNA Codons & Amino Acids• What amino acid does the AUG codon code for?
mRNA Codons & Amino Acids
Translation• What is translation?
– process of reading mRNA codons to synthesize proteins
• with help of tRNA (transfer RNA) & ribosomes
• When and where does translation occur?– when mRNA from
the nucleus joins with ribosomes in the cytoplasm
1. mRNA comes from the nucleus & joins with ribosome in cytoplasm
2. Ribosome “reads” the codon until it finds the start codon (AUG)
3. tRNA (with complementary anticodon) joins to the mRNA codon
– tRNA carries the amino acid specified by the mRNA codon
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.swf
Steps of Translation
4. Ribosome moves along mRNA reading each codon until “stop” codon is reached
– tRNA transfers each amino acid to ribosome
5. Amino acids line up & bond to form polypeptide
6. Polypeptide chain forms protein (3-D)– Shape of protein is related to its function
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.swf
Steps of Translation
tRNA anticodon
DNA codon
mRNA codon
transcription translation
mRNA codon
Let’s Practice… What would the mRNA strand made during
transcription look like?
What amino acids would be coded for using this mRNA?
What would the anticodons on the tRNA be for each amino acid?
Let’s Practice…• Given the DNA sequence below, determine the:
1. mRNA codon sequence
2. amino acids that would be coded for by each codon
3. anticodons on each tRNA which allowed it to “bring” that amino acid (using the mRNA codons)
• DNA = TAC CCA TTG GAT CCG ACT
• mRNA codon = • amino acid = • tRNA anticodon =
AUG GGU AAC CUA GGC UGA
Met/start Gly Asn Leu Gly stop
UAC CCA UUG GAU CCG ACU
Mutations• What are mutations?
– mistakes/changes in a gene on a chromosome• can occur spontaneously
during replication• can be caused by mutagens
(such as radiation, high temperatures, or chemicals)
• often corrected, but not always– once occurs, copied as if correct– can cause different protein
Types of Mutations• What are the 3 types of mutations?
1. deletion• What happens in a deletion mutation?
– a nucleotide (base) is left out» causes “frame shift” ~moves nucleotides after the deletion to the left causing a
different protein to be formed
2. insertion• What happens in an insertion mutation?
–a nucleotide (base) is added in» causes “frame shift” ~moves nucleotides after the insertion to the right causing a
different protein to be formed
Types of Mutations
Types of Mutations
3. point (base substitution)• What happens in a point
mutation?–one nucleotide (base) is
replaced by another» affects only 1 amino acid &
does not cause “frame shift”
» sometimes amino acid & protein can be the same (“silent mutation”)
» protein may be able to function normally or nearly normally
Sickle Cell Mutation• What does the sickle cell mutation do?
– affects hemoglobin (protein in blood carries O2)
• What type of mutation causes sickle cell?– point mutation
• changes one amino acid– changes the protein
“Silent” mutation (no change in polypeptide)
(point mutation)