Unit 6: DNA & Protein Synthesis Ch. 28: DNA—Life’s Code DNA = Deoxyribonucleic Acid.
Post on 03-Jan-2016
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Describing DNA
• Who first described DNA?– James Watson &
Francis Crick
• What shape did they use to describe DNA?– double helix
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, or cytosine
A Model of DNA• Is a DNA molecule single-
stranded or double stranded?– double-stranded
• What parts make up the strands?– “backbone”/sides
• sugars & phosphates
– “rungs”/steps• nitrogenous bases
– always paired as:» A - T
» G - C
DNA Structure Video
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. DNA molecule unzips between bases forming two “parent strands”
What are the steps in DNA replication?
• 2. The “parent” strands act as templates (guides)…
– “new” nucleotides are paired up with the complementary base on the “parent” strand
• forming a complementary daughter strand– ex. “G” pairs with “C”
What are the steps in DNA replication?
T
C
• 3. Now, there are 2 exact copies of the original DNA molecule.– & when the 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?
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
Transcription• What is transcription?
– DNA code is “transcribed”/copied into mRNA codons
• When does transcription happen?– when a protein is
needed
• Where does transcription happen?– in the nucleus
Transcription• What does transcription make?
– A single-stranded messenger RNA (mRNA)• made from instructions/order of the bases in the DNA
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf
Transcription• What are the steps in
transcription?1. DNA unzips
2. free RNA nucleotides pair w/ exposed bases on DNA• until “stop codon” is
reached
3. mRNA separates from DNA• & moves out of nucleus
– to ribosome in cytoplasm
4. DNA strands rejoin
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?
Translation• What is translation?
– process of reading mRNA codons to synthesize proteins
• with help of tRNA (transfer RNA) & ribosomes
• When does translation occur?– when mRNA from the
nucleus joins with ribosomes
• Where does translation happen?– at 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 brings each amino acid
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
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
• What type of mutation causes sickle cell?– A point mutation
• changes one amino acid– changes the protein
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