Download Biology 30 Unit C 1 Mr. R. Peebles Biology 30

Biology 30
Unit C – Molecular Genetics: DNA / Protein Synthesis
General Outcome C3: Students will explain classical genetics at the molecular level.
A. DNA
• deoxyribonucleic acid
• the simplest forms of life all contain DNA
• it is the only molecule that we know can replicate itself
• DNA makes up the genes (100 000) found on the chromosomes
• it provides continuity of life from generation to generation
• it is responsible for cells ability to repair itself and reproduce
• it may be changed by mutation but will still function
• discovered in 1953 by James Watson & Francis Crick (nobel prize in 1962)
B. Structure of the DNA
• the molecule is a double helix shape - like a twisted rope ladder
• composed of two distinct parts
a) sides of the molecule (ladder)
b) rungs of the molecule (ladder)
a) Sides of the Ladder
• made up of alternating groups of deoxyribose sugar and phosphate groups
• these groups are joined by a strong molecular bond (covalent)
b) Rungs of the Ladder
• consist of two bases joined by a weak hydrogen bond
• the base pairs are joined to the sugar not the phosphate group
• two major types of bases:
1. Purines - Adenine (A) or Guanine(G)
2. Pyrimidines - Thymine (T) or Cytosine
• these bases will only bond in one way
A-T and C-G
• this is called complimentary base paining
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• a nucleotide is composed of three parts
1. a sugar
2. a phosphate
3. a nitrogen base
• this is the main repeating unit of the DNA
C. Replication
• DNA must be able to replicate itself completely so that the cell can divide (mitosis) and
2 identical cells can be formed
• each replicated strand is a duplicate of the original
• Also termed Semiconservative replication
– The original parent strands are conserved, original molecule is not
Process
1. The hydrogen bond between bases splits and
the two strands become ‘unzipped’ by the
enzyme helicase
2. RNA Primers initiate the formation of new
sections of DNA
3. New nucleotides are attached by DNA
polymerases III
• Nucleotides originating from protein found
in food
4. RNA primers are removed by DNA
polymerases I
(replacing with DNA
instead of RNA)
5. Ligase joins any gaps
between sugarphosphate molecules
6. When complete, 2
identical DNA
molecules are
present.
•
DNA polymerase II ‘proofread ‘the new strands looking for mismatched pairs to fix
D. Recombinant DNA
• this is the technology of taking genetic material from one organism and placing it in
another organism
• may also be called gene splicing or genetic engineering
• enzymes have been discovered which will cut apart DNA (restriction enzymes) and
will glue DNA together (ligase enzymes)
• has become a multibillion dollar industry in Biotechnology
• one example - insulin - produced genetically by using E. coli bacteria
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E. RNA
• ribonucleic acid
• another type of nucleic acid used during protein
synthesis
• it differs from DNA in the following ways
1. the sugar is ribose
2. it is a single strand
3. the base thymine (T) is replaced by uracil (U)
AUCG vs. ATCG
4. RNA may be found in the nucleus or the cytoplasm but DNA never leaves the nucleus
• there are three types of RNA
a) mRNA - messenger RNA - copies a potion of DNA and takes it to the ribosome
b) tRNA - transfer RNA - picks up amino acids and takes them to the ribosome
c) rRNA - ribosomal RNA - makes up the ribosome
F. Protein
• found in every living cell
• used as building blocks for all parts of the cell (membranes, organelles, nucleic acids)
• cells produce their own protein
• by linking defective enzymes(protein molecules) to genetic mutations scientists Beadle
& Tatum came up with the ‘one gene-one enzyme’ gene theory
• each protein is specific to that organism
• raw materials are amino acids - 20 different amino acids used by humans
• a protein is a long chain of amino acids
• the blueprint for the protein is contained in the nucleus (DNA) and the protein is
manufactured in the ribosome’s
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•
protein synthesis involves two processes
1. transcription
2. translation
Protein Synthesis
• there can be thousands of amino acids in a single protein chain
• the sequence of amino acids determines the type of protein
• if even one amino acid is out of order, it may have very severe results
• e.g. sickle-cell anemia - 1 valine is substituted by a glutamic acid
• each set of three bases on the DNA molecule codes for one specific amino acid
• this set of three bases on the mRNA is called a codon
• there are 64 different combinations of 3 bases - **see handout **
1. Transcription
• since DNA cannot leave the nucleus, a copy of a gene must be made
• Three distinct phases
A) Initiation
! RNA polymerase binds to segment to be transcribed and opens the double helix
! RNA binds to a region in front of a gene termed the promoter (section of adenine and
thymine bases)
! Indicates which strand to transcribe and where RNA polymerase should start
B) Elongation
! The formation of mRNA by RNA polymerase
! Similar to DNA replication except the mRNA contains uracil instead of thymine
C) Termination
! RNA polymerase stops transcribing a gene when it reaches the termination sequence
of bases
• Transcription stops and the mRNA strand peels of the DNA and moves to a ribosome
in the cytoplasm
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•
after the copy is made, the DNA molecule zips back up
2. Translation
• this is the actual reading of the code and the
buildup of the protein
• the mRNA chain moves along the ribosome
• tRNA which has on one end an individual base
triplet segment - anticodon - picks up the
proper amino acid and delivers them to the
ribosome
• anticondons contain complimentary base
sequences for codons
• i.e. cysteine – Codon = UGU, anticodon
= ACA
• Similar three stages as transcription:
A) Initiation
! Ribosome binds to the mRNA at the start codon
! start codon (AUG = methionine) indicates
where translation begins, therefore all
polypeptides begin with methionine (Met)
B) Elongation
! A tRNA for Met binds first to the start codon at one of two binding sites in RNA (P site)
carrying the required amino acid.
! The next tRNA for the following codon enters the other binding site (A site) carrying the
need amino acid
! A peptide bond will form between the two amino acids.
! The ribosome will move down one codon, so that the second tRNA is now in the P site
and the A site is empty ready to accept the next tRNA.
! Met-carrying tRNA is released leaving behind the amino acid Met- bonded to the
adjacent amino acid.
! as the ribosome moves along the mRNA, more and more amino acid are joining by
peptide bonds to form the protein
! see p. 673 Nelson
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C) Termination
! The ribosome will reach a stop codon
! Stop codons (UAA, UAG, UGA) do not code for amino acids and cause the protein
synthesis to stop, releasing the finished protein
•
the mRNA may be read hundreds to times forming many copies of the same protein
Assignment
• Using well labelled diagrams depict the formation of protein to meet the following
requirements:
1. Include the following diagrams
a) nucleus - showing the mRNA
b) cytoplasm - showing at least 4 sequential steps in the formation of the protein chain
2. The protein must be compose of the following amino acids
1. Alanine
6. aspartate
2. Serine
7. serine
3. Threonine
8. phenylalanine
4. Valine
9. proline
5. Histidine
10. glutamate
3. Include initiator and terminator codons
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G. Sorting and Analyzing DNA
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Gel electrophoresis is used to separate molecules according to their mass and charge
It is used to separate fragments
produced by using restriction
enzymes
A solution that contains DNA
fragments is applied to one end of
a gel
An electric current is then passed
through the gel
One end of the gel will develop a
positive electric charge, the other
a negative electric charge.
DNA will be attracted to the
positive charge as it is negatively
charged
Smaller fragments move more
frequently
Fragments separate into a pattern
of bands, a DNA fingerprint
Different samples that display
similar DNA fragments provided
evidence of inheritance
Used in crime scene investigation and paternal cases
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http://learn.genetics.utah.edu/units/biotech/gel/
Your Assignment: Thought Lab 18.5 – Reading a DNA Fingerprint
p. 651 MHR
H. Tracing Ancestry
! The cytoplasm in a zygote is donated by the ovum
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The sperm contributes essentially no cytoplasm, therefore no cytoplasmic
organelles.
The DNA in the nuclei of your cells is made of equal combination from the
parents, your mitochondrial DNA (mtDNA) is genetically identical to your
mothers mtDNA.
Both mtDNA and chloroplast DNA is independent of the nuclei DNA
If two people have identical mtDNA sequences, they likely share a relatively
recent maternal ancestor.
By comparing mtDNA of different living people scientists can deduce lineage
patterns that reveal prehistoric relationships among human populations
As well as evolutionary paths of animals and plant species can be determined.
I. Mutations
Three basic forms of mutations possible
1) point mutations – a mutation at a specific base pair
!
May or may not change the sequence of amino acids
a. Silent mutation – a change in base pairs that does not result in a change in an amino
acid ie. Cysteine UGU to UGC
b. Missense mutation – a change in base sequence which results in altering the codon
leading to a different amino acid i.e. sickle cell anemia
c. Nonsense mutation – a change in base sequence that cause a stop codon to replace
an amino acid codon.
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Results in a fragmented polypeptide.
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Are often lethal to the cell.
2) gene mutation – a mutation that changes the coding for amino acids
a. Deletion – one or more nucleotides are removed from the DNA sequence
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may alter more than one amino acid at a time resulting in significant changes to
the protein
b. Insertion – one or more nucleotides are inserted to the DNA sequence
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Results in different amino acid sequences to be translated
3) chromosome mutation – mutation of large segments of DNA and is seen at the
chromosomal level
a. Translocation – the relocation of groups of base pairs from one part of the genome to
another
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occurs usually between nonhomologous chromosomes i.e. some forms of
leukemia
b. Inversion – section of chromosome that has reversed its orientation in the
chromosome
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! Mutations may arise form either
o spontaneous mutations – malfunction of DNA polymerase I misses a base or
two resulting in a point mutation
o Mutagenic agents – cause induced mutations i.e. UV radiation, x-rays
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