Download Protein Synthesis - Elgin High School

DNA
• An organism’s DNA ultimately controls that
organism’s traits.
– All living things contain proteins
– Most chemical reactions that construe metabolism are
controlled by enzymes – a type of protein
– Proteins can only be made if the correct instructions are
present.
– DNA contains the instructions to create every protein
that an organism will require
DNA
• A DNA molecule is a polymer made
up of many smaller units called
nucleotides, linked together.
• A nucleotide is made up of three parts:
– Simple sugar (in the case of DNA, the
sugar is deoxyribose)
– Phosphate group
– Nitrogen base
• Every nucleotide is exactly like every
other nucleotide, except for the
nitrogen base.
DNA
• There are 4 possible nitrogen
bases in DNA
–Guanine (G)
–Cytosine (C)
–Adenosine (A)
–Thymine (T)
DNA
• The structure of DNA is similar to a twisted
ladder
– The sides of the ladder are the phosphate group
and simple sugar
– The ‘rungs’ consist of two, complimentary bases,
held together by weak hydrogen bonds
– The bases only match up in one way….
A with T
C with G
Hydrogen bond
1 nucleotide
Sugar
A
T
C
G
C
D
G
Phosphate
G
C
T
A
T
A
A
T
N
A
Nitrogen Base
DNA
• The only difference between one nucleotide and
another is the nitrogen base it contains.
• There are only four nitrogen bases.
• Every protein is made up of combinations of these
four bases.
• So, the
sequence of the nucleotides (or
more specifically, of the nitrogen bases)
is what determines the protein.
DNA Replication
• When a cell makes a copy of itself (either thru
mitosis or meiosis), it must make a copy of the DNA
or the new cell won’t have the instructions it needs.
DNA replication is the process of copying
DNA so that you have two identical strands.
• One strand will move on to the new cell, and one
will remain with the original cell.
DNA Replication
• First, the double helix has to unwind. DNA helicases
(enzymes) open the double helix by breaking the weak
hydrogen bonds, splitting it into two separated strands.
– Additional proteins attach to each strand keeping them from
rejoining.
• Then enzymes called DNA polymerases move along the
strands, adding free nucleotides to the exposed bases,
following the base-pairing rules, thus forming two identical
double helixes.
• Once all of the DNA has been copied, the DNA polymerases
detach, and you have two identical strands of DNA – each is
made up of one original strand and one new strand.
BIO – DNA Replication
• DNA polymerases have a proofreading role – they can only
move on to the next nucleotide to the growing strand if the
previous nucleotide is paired correctly.
• If a mistake was made, the DNA polymerase can backtrack
and fix it.
• Thus, errors in DNA replication are about 1 per billion
nucleotides.
• It takes about 8 hours to completely replicate a human
chromosome (each is made up of about 100 sections of
100,000 nucleotides each)
DNA Replication
• Given the DNA strand G-A-T-T-A-C-G-C-C-A,
what would be the complimentary copy made DNA
Replication?
Protein Synthesis
• DNA is the instructions to make proteins…..
• A protein is a polymer, made up of many amino
acids….there are only about 20 different amino
acids, so the DNA gives the correct sequence for
these amino acids to link up to create each different
protein.
Protein
Synthesis
• What organelle
makes proteins?
• And where are
the instructions?
• Somehow, the instructions from the DNA have to
get out of the nucleus, to the ribosomes, so that the
proteins can be made.
Protein Synthesis
• The processes involved in creating a new protein
from the instructions on the DNA is called protein
synthesis.
• Nucleic acids called RNA are required for protein
synthesis.
– RNA is structurally like DNA with a few differences:
• The simple sugar is called ribose
• It is made up of only one strand
• RNA does not have thymine, but rather has uracil (U), which
matches up to adenosine. (A)
Protein Synthesis
• Protein synthesis begins by
making a copy of the DNA, a
process called trancription.
– The DNA strand uncoils like
it did for replication
– mRNA (messenger RNA) links to
the nucleotides link to the open
strand, making a complimentary copy of the DNA.
• Always read from the 5’ toward the 3’ end of the DNA strand.
• The DNA strand is read in a series of 3, called triplets.
• The complimentary mRNA strand is called a codon.
DNA triplet
mRNA codon
T-A-C A-T-T G-A-T A-A-T
A-U-G U-A-A C-U-A U-U-A
Protein Synthesis
• The mRNA leave the nucleus and moves to a ribosome where it binds to a rRNA
(ribosomal RNA), reads the instructions and ensures that the correct amino acids
are brought and assembled.
• The process of reading the mRNA to assemble the correct amino acids in order is
called translation.
*****You read the codon to determine*****
which amino acid you need.
Protein Synthesis
• tRNA brings the correct amino acid to the mRNA
codon…..the tRNA anticodon is the complimentary
sequence to the mRNA codon.
–
–
–
–
DNA triplet
T-A-C A-T-T
mRNA codon
A-U-G U-A-A
Amino Acid:
tRNA antiocodon:
G-A-T
C-U-A
A-A-T
U-U-A
Codons:
AUG
GGC
AGC
UUA
GUA
GCC
AUC
AAC
UAA
Codons:
AUG
GAU
ACC
CCU
CAA
GAC
UGA
Protein Synthesis
• There is a ‘start’ codon sequence, that is
recognized as the beginning of a new set of
instructions, just like a capital letter often
indicates a new sentence.
– AUG is the ‘start’ codon, and codes for
methionine.
– So, all proteins will have methionine as the first
protein in their sequence.
– There are also various ‘stop’ codons that tell you
that the sequence is ended.
Protein Synthesis
Summary
• Transcription:
– Where?
– What?
In the nucleus
DNA triplet copied onto a mRNA codon
• Translation:
– Where?
At the ribosomes
– What?
rRNA reads the mRNA codon and the
tRNA anticodons bring the appropriate
amino acid
BIO – Protein Synthesis
• With few exceptions, the genetic code is the same in
all organisms – GUC codes for the amino acid
valine in bacteria, eagles, plants and
people…..therefore it is often described as almost
universal, giving rise to the idea that all life-forms
have a common evolutionary ancestor.
• Exceptions include the way cell organelles with their
own DNA (mitochondria, chloroplasts) and how
some microscopic protists read the ‘stop’ codons.
Chromosomal Problems
• Mutations can also happen when the chromosomal
structure breaks during meiosis. There are five
types:
– Deletion Mutation: Piece of chromosome breaks off
completely – often fatal
Chromosomal Problems
– Insertion Mutation: A chromosome piece breaks off
from one chromosome, and inserts itself into another one.
Chromosomal Problems
– Duplication Mutation: Chromosome fragment attaches
to its homologous chromosome, so that it now carries
two copies of a certain set of genes.
Chromosomal Problems
– Inversion Mutation: A chromosome piece reattaches to
the original chromosome, but in reverse.
Chromosomal Problems
– Translocation Mutation: A chromosome piece attaches
to a nonhomologous chromosome…..similar to crossing
over, except that it happens between two nonhomologous
chromosomes.
Repairing DNA
•
Mutagens are agents that may cause a change in DNA.
– Include exposure to radiation, chemicals, high temperatures, etc.
• Radiation and perhaps high temps, cause havoc because of the
high amounts of energy. Radiation that passes thru your cells
acts like little bullets, cutting the DNA in places.
• Some chemicals that are highly reactive, may react with the
DNA molecule, causing changes.
– The damage may kill the cell which is preferable.
– The cell may live but the if the damage is be in one of the
introns, then no harm is done
– The cell may live, and if the damage is to one or more
sequences that code for proteins, then either needed
proteins are not made or proteins you do want are –
resulting in problems!
Repairing DNA
• There are fail-safes in the cells to avoid mutations.
– During replication, there are enzymes that act as a spellcheck to ensure that the DNA strand is copied correctly.
– When DNA is damaged by mutagens, those same
enzymes and others act to again proofread the strand and
replace incorrect sequences with free nucleotides.
• The greater the exposure to the mutagen, the more
likely the chance that a mistake will sneak by.