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Protein Synthesis and Mutation (Translation (role of tRNA (anticodon at…
Protein Synthesis and Mutation
Protein Synthesis + Transcription
DNA never leaves nucleus
ribosomes are in cytoplasm
two processes necessary to make a protein from the information in DNA are transcription and translation
transcription happens in the nucleus and uses the DNA sequence to make an RNA molecule
RNA then leaves the nucleus and goes to the cytoplasm where translation occurs on a ribosome and produces a protein
central dogma of molecular biology deals with the detailed residue-by-residue transfer of sequential information
central dogma of molecular biology describes the fundamental process that makes us all different
we all have the same genes and proteins
different alleles for each gene
many of our proteins are slightly different - work slightly differently
if all proteins acted the same then we would all be the same
proteins are different because we have different alleles
transfer of sequence information
polymers that comprise DNA, RNA and amino acids are linear polymers
each monomer is connected to two more monomers
sequence of their monomers encodes genetic information
with three biopolymers (DNA, RNA, and protein), there could be as many as nine potential types of transfers
central dogma classes into 3 groups of 3
three general transfers
three special transfers
three unknown transfers
transfers of information described by the central dogma based on the order of monomers that make this genetic information
polymer's DNA or RNA sequence is used as template for the construction of another polymer with a sequence that is entirely dependent on the original polymer's sequence
reverse transcription
reverse transcription is the transfer of information from RNA to DNA
occurs in retroviruses, such as HIV
telomerase: a reverse transcriptase that uses an RNA intermediate to elongate the 3' end of DNA strands in the telomere regions after each replication cycle
retroviruses are RNA viruses that are duplicated in a host cell by using reverse transcriptase enzyme to produce DNA from its RNA genome
also occurs with retrotransposons and during telomere synthesis in eukaryotes
retrotransposons are self-replicating segments of eukaryotic genomes that use reverse transcriptase to move from one position in the genome to another via a RNA intermediate
RNA replication
RNA replication is the copying of one RNA to another
many RNA viruses copy their RNA using RNA replication
RNA-dependent RNA polymerases = enzymes specific for this process, as opposed DNA-dependent RNA polymerase, which catalyzes the transcription of RNA from a DNA template
RNA silencing refers to mechanisms of gene silencing
antisense RNA molecule is a single-stranded RNA that is complementary to a mRNA strand transcribed within a cell
Genetic Code
gene coded through genetic code
A, C, G, U—in an mRNA chain
genetic code consists of the sequence of nitrogen bases
four bases make up the “letters” of the genetic code
codons: etters are combined in groups of three to form code “words"
codon stands for one amino acid, unless it codes for a start or stop signa;
20 common amino acids in proteins
64 possible codons
reading the genetic code
codon AUG codes for the amino acid methionine
start codon that begins translation
reading frame is the way the letters are divided into codons
UAG, UGA, and UAA are all stop codons
known as termination codons
characteristics of the genetic code
genetic code is universal - all living organisms use the same genetic code, which shows that all organisms share a common evolutionary history
genetic code is unambiguous - each codon codes for just one amino acid
genetic code is redundant - most amino acids are encoded by more than one codon
Translation
RNA --> Protein
translation is the transfer of the genetic instructions in RNA to a protein made of amino acids
translation uses the products of transcription, mRNA, tRNA, and rRNA, to convert the mRNA sequence into a polypeptide according to the genetic code
mRNA moves from the nucleus to the cytoplasm to interact with a ribosome
ribosome is site of translation
translation proceeds in three phases: initiation, elongation and termination
ribosomes are composed of two subunits, a small subunit and a larger subunit
during translation the tRNA molecules are literally “inside” the ribosomal subunits, as they sit on the mRNA strand
when tRNAs come to the ribosome, adjacent amino acids are brought together and allow the ribosome to catalyze the formation of the peptide bond between amino acids
ribosome has three tRNA binding sites: the A site, the P site, and the E site
A site binds a tRNA with an attached amino acid
P site contains the tRNA with the growing polypeptide chain attached
the E site contains the tRNA that no longer has an attached amino acid
single mRNA can be translated simultaneously by multiple ribosomes
role of tRNA
transfer RNAs or tRNAs bring or tansfer the proper amino acid to the ribosome based on the genetic code
anticodon at the bottom of the tRNA molecule binds to the codon on the mRNA
codon on the mRNA is specific for an amino acid or stop codon
only one amino acid can be attached to a tRNA, based on that tRNA's anticodon
there are 61 separate codons that can bind to anticodons -- 61 different tRNAs in a cell
covalent attachment of an amino acid to the tRNA is catalyzed by enzymes called aminoacyl-tRNA syntheses through a process called aminoacylation
there is a single aminoacyl tRNA synthetase for each amino acid
initiation in prokaryotes
nitiation of translation in prokaryotes involves the assembly of the ribosome and addition of the first amino acid, methionine
the 30S ribosomal subunit attaches to the mRNA
initiation in eukaryotes
initiation of protein translation in eukaryotes is similar to that of prokaryotes with some minor modifications
the 5' cap and 3' poly(A) tail are involved in the recruitment of the ribosome
elongation
similar between prokaryotes and eukaryotes
as translation begins, the start tRNA is sitting on the AUG codon in the P site of the ribosome, so the next codon available to accept a tRNA is at the A site.
elongation proceeds after initiation with the binding of an tRNA to the A site
termination
occurs when the ribosome comes to one of the three stop codons, for which there is no tRNA
a protein called a release factor binds to the A site
events following protein synthesis includes post-translational modification of the peptide chain and folding of the protein into its functional conformation
two or more polypeptides may interact with each other, forming a functional protein with a quaternary structure
Mutation
all genes have alternative forms causing the protein to have a different function
mutation: a change in the DNA or RNA sequence, and many mutations result in new alleles
some have no effect
others are beneficial and others are harmful
evolution could not take place without the genetic variation that results from beneficial mutations
change in a DNA or RNA sequence
new genes
large mutations form new genes
some duplicate large sections of DNA - major source of genetic material for new genes
gene families have domains within the protein with a particular and independent function
duplication is the way that the gene families form
alu sequences
DNA sequences that can translocate around the genome comprise a significant fraction of the genetic material of plants and animals
million copies of the Alu sequence are present in the human genome
makes transposon equivalent to just under 11% of the human genome
elements are about 300 base pairs long and are classified as short interspersed elements among the class of repetitive DNA elements
elements are derived from the signal recognition particle RNA
tRNA is the RNA component of the signal recognition particle ribonucleoprotein complex
sequences are repetitive elements that form a significant part of the human genome
new chromosomes
it is possible for two chromosomes to fuse
the hominids that arose after the split from the orangutans had two chromosomes fused to produce chromosome 2 in humans
occurred to keep two populations from interbreeding
would be genetically incompatible
Mutation Causes
can occur spontaneously or there can be a cause
spontaneous mutation can just happen
occur during normal cellular processes
due to a mistake during DNA replication or transcription
mutagenesis: process by which the genetic information of an organism is changed in a stable manner
result in mutation
harmful: lead to cancer and heritable diseases
beneficial: driving force of evolution
mutagen
known as an induced mutation, caused by an environmental factor
mutagens include chemicals inhaled while smoking and radiation
Hermann Muller first demonstrated the effects of mutations in 1927
observable changes in chromosomes
induced mutagenesis by irradiating fruit flies with X-rays
Mutation Effects
majority of mutations have no negative or positive effects on the organism in which they occur
alled neutral mutations
silent point mutations
neutral because they do not change the amino acids in the proteins they encode
if a cell’s DNA is permanently damaged and cannot be repaired, then the cell is prevented from dividing
beneficial mutations
lead to new versions of proteins that help organisms adapt to changes in their environment
increase an organism’s changes of surviving or reproducing, so they are likely to become more common over time
mutations in bacteria that allow them to survive in the presence of antibiotic drugs - lead to antibiotic-resistant strains of bacteria
have no effect on the organism because they are repaired before protein synthesis occurs
cells have multiple repair mechanisms to fix mutations in DNA
harmful mutations
any random change in a gene's DNA will result in a protein that does not function normally or may not function at all
like making a random change in a complicated machine such as a car engine
engine won't imrpove
genetic disorder: disease caused by a mutation in one or a few genes
cystic fibrosis