gene expression
- the process by which DNA directs the synthesis of proteins
- includes two stages: transcription and translation
Archibald Garrod (1902)
- first to suggest that genes dictate phenotypes through enzymes that catalyze specific chemical reactions in the cell
- alkaptonuria (black urine)
George Beadle, Boris Ephrussi (1930s)
- speculated that in Drosophila, each of the various mutations affecting eye color blocks pigment synthesis at a specific step by preventing production of the enzyme that catalyzes that step
George Beadle, Edward Tatum
- Bread mold (Neurospora crassa)
- Bombarded Neurospora with X-rays to cause genetic changes and then looked among the survivors for mutants that differed in their nutritional needs from the wild-type bread mold.
- They identified mutants that could not survive on minimal medium because they were unable to synthesize certain essential molecules from the minimal ingredients.
What is the difference between RNA and DNA?
- RNA contains ribose instead of deoxyribose as its sugar
- has the nitrogenous base uracil rather than thymine
transcription
The synthesis of RNA using a DNA template
messenger RNA (mRNA)
A type of RNA, synthesized using a DNA template, that attaches to ribosomes in the cytoplasm and specifies the primary structure of a protein. (In eukaryotes, the primary RNA transcript must undergo RNA processing to become mRNA)
translation
The synthesis of a polypeptide using the genetic information encoded in an mRNA molecule. There is a change of "language" from nucleotides to amino acids.
ribosomes
molecular complexes that facilitate the orderly linking of amino acids into polypeptide chains
primary transcript
An initial RNA transcript from any gene; also called pre-mRNA when transcribed from a protein-coding gene.
Who came up with the concept of central dogma and explain what it is?
- Francis Crick (1956)
- Genetic information flows from DNA to RNA to protein
triplet code
A genetic information system in which sets of three-nucleotide-long words specify the amino acids for polypeptide chains
template strand
The DNA strand that provides the pattern, or template, for ordering, by complementary base pairing, the sequence of nucleotides in an RNA transcript.
codons
A three-nucleotide sequence of DNA or mRNA that specifies a particular amino acid or termination signal; the basic unit of the genetic code.
reading frame
On an mRNA, the triplet grouping of ribonucleotides used by the translation machinery during polypeptide synthesis.
RNA polymerase
an enzyme that links ribonucleotides into a growing RNA chain during transcription, based on complementary binding to nucleotides on a DNA template strand.
what are the stages of transcription?
- initiation
- elongation
- termination
promoter
A specific nucleotide sequence in the DNA of a gene that binds RNA polymerase, positioning it to start transcribing RNA at the appropriate place.
terminator
In bacteria, a sequence of nucleotides in DNA that marks the end of a gene and signals RNA polymerase to release the newly made RNA molecule and detach from the DNA.
transcription unit
A region of DNA that is transcribed into an RNA molecule.
start point
In transcription, the nucleotide position on the promoter where RNA polymerase begins synthesis of RNA.
transcription factors
A regulatory protein that binds to DNA and affects transcription of specific genes.
transcription initiation complex
The completed assembly of transcription factors and RNA polymerase bound to a promoter.
TATA box
A DNA sequence in eukaryotic promoters crucial in forming the transcription initiation complex.
polyadenylation signal sequence
...
RNA processing
modification of RNA primary transcripts, including splicing out of introns, joining together of exons, and alterations of the 5' and 3' ends.
5' cap
A modified form of guanine nucleotide added onto the 5' end of a pre-mRNA molecule.
poly-A tail
A sequence of 50-250 adenine nucleotides added onto the 3' end of a pre-mRNA molecule.
RNA splicing
After synthesis of a eukaryotic primary RNA transcript, the removal of portions of the transcripts, including splicing out of introns, joining together of exons, and alteration of the 5' and 3' ends.
introns
A noncoding, intervening sequence within a primary transcript that is removed from the transcript during RNA processing; also refers to the region of DNA from which this sequence was transcribed.
exons
A sequence within a primary transcript that remains in the RNA after RNA processing; also refers to the region of DNA from which this sequence was transcribed.
spliceosome
A large complex made up of proteins and RNA molecules that splices RNA by interacting with the ends of an RNA intron, releasing the intron and joining the two adjacent exons.
ribozymes
An RNA molecule that functions as an enzyme, such as an intron that catalyzes its own removal during RNA splicing.
alternative RNA splicing
A type of eukaryotic gene regulation at the RNA-processing level in which different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns.
domains
A discrete structural and functional region of a protein.
transfer RNA (tRNA)
functions as a translator between nucleic acid and protein languages by carrying specific amino acids to the ribosome, where they recognized the appropriate codons in the mRNA.
anticodon
A nucleotide triplet at one end of a tRNA molecule that base-pairs with a particular complementary codon on an mRNA molecule.
aminoacyl-tRNA synthetases
An enzyme that joins each amino acid to the appropriate tRNA.
wobble
Flexibility in the base-pairing rules in which the nucleotide at the 5' end of a tRNA anticodon can form hydrogen bonds with more than one kind of base in the third position (3' end) of a codon.
ribosomal RNAs (rRNA)
RNA molecules that, together with proteins, make up ribosomes; the most abundant type of RNA
P site
One of a ribosome's three binding sites for tRNA during translation. The P site holds the tRNA carrying the growing polypeptide chain. (P stands for peptidyl tRNA)
A site
The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. (A stands for aminoacyl tRNA)
E site
The E site is the place where discharged tRNAs leave the ribosome. (E stands for exit).
signal peptide
A sequence of about 20 amino acids at or near the leading (amino) end of a polypeptide that targets it to the endoplasmic reticulum or other organelles in a eukaryotic cell.
signal-recognition particle (SRP)
A protein-RNA complex that recognizes a signal peptide as it emerges from a ribosome and helps direct the ribosome to the endoplasmic reticulum (ER) by binding to a receptor protein on the ER.
polyribosomes
A group of several ribosomes attached to, and translating, the same messenger RNA molecule.
mutations
A change in the nucleotide sequence of an organism's DNA or in the DNA or RNA of a virus.
point mutations
A change in a single nucleotide pair of a gene.
nucleotide-pair substitution
A type of a point mutation in which one nucleotide in a DNA strand and its partner in the complementary strand are replaced by another pair of nucleotides.
silent mutation
A nucleotide-pair substitution that has no observable effect on the phenotype; for example, within a gene, a mutation that results in a codon that codes for the same amino acid.
missense mutations
A nucleotide-pair substitution that results in a codon that codes for a different amino acid.
nonsense mutations
A mutation that changes an amino acid codon to one of the three stop codons, resulting in a shorter and usually non-functional protein.
insertions
A mutation involving the addition of one or more nucleotide pairs to a gene.
deletions
A mutational loss of one or more nucleotide pairs from a gene.
frameshift mutation
A mutation occurring when nucleotides are inserted in or deleted from a gene and the number inserted or deleted is not a multiple of three, resulting in the improper grouping of the subsequent nucleotides into codons.
mutagens
A chemical or physical agent that interacts with DNA and can cause a mutation.