Chapter 17 Flashcards
what form is the content of DNA in?
specific sequences of nucleotides that is inherited by an organism and leads to specific traits by dictating the synthesis of proteins
Gene Expression
the process by which DNA directs protein synthesis
two stages: 1) transcription 2) translation
2 stages of gene expression
1) transcription
2) translation
the study of metabolic defects …
provide evidence that genes specify proteins
Archibald Garrod
first to suggest that genes dictate phenotypes through enzymes that catalyze specific chemical reactions in the cell
1909
Beadle and Tatum
developed the “one gene-one enzyme theory”
cause bread mold (neurospora cassia) to mutate with X-rays creating mutants that could not survive on minimal medium
1930s
“one gene-one enzyme theory”
states that the function of a gene is to dictate the production of a specific enzyme
DNA => RNA => protein (enzymes)
* not all proteins are enzymes {i.e. actin and tubuline}
Genes code for …
polypeptide chains or for RNA molecules
RNA is…
the bridge to proteins (from DNA)
Transcription
the synthesis of RNA under the direction of DNA that produces messenger RNA (mRNA)
the DNA-directed synthesis of RNA
the conversion of DNA into RNA
Translation
the actual synthesis of a polypeptide which occurs under the direction of mRNA and occurs on ribosomes
the conversion of nucleotide (RNA) into polypeptide
Gene expression in Prokaryotes
lack a nuclei, so transcription and translation occur together
DNA => RNA => protein
Ribosomes help: attach to the leading end of mRNA and start translating while transcription is still in progress (to synthesize proteins)
Ribosomes
attach to the leading end of mRNA and start translating while transcription is still in progress (to synthesize proteins)
Gene expression in Eukaryotes
RNA transcripts (primary transcripts) are made in the nucleus are modified before becoming true mRNA
mRNA will leave the nucleus and be translated by the ribosomes
mRNA
can leave the nucleus and be translated by the ribosomes
Genetic Code: Cells are governed by a cellular chain of command
DNA => RNA => protein
Genetic information is encoded
as a sequence of non overlapping base triplets or codons
DNA is a series of …
3-nucleotides with instructions for RNA and a specific polypeptide chain
Codons
3 mRNA nucleotides that refer to a specific amino acid
During transcription, the gene determines…
the sequence of bases for an entire strand of mRNA
Condon in mRNA
either translated into an amino acid or serves as a translational stop signal (codon).
must be read in the correct reading frame for the specified polypeptide to be produced
Start Codon
AUG
Stop Codon
UGA
UAG
UAA
Components of Transcription
RNA synthesis is catalyzed by the enzyme RNA polymerase, which opens the DNA strands and hooks together the RNA nucleotides (5’ - 3’)
RNA follows the same base-pairing rules as DNA, excepted that in RNA, uracil substitutes for thymine
3 stages of Transcription
1) Initiation (find DNA)
2) Elongation (synthesis of growing RNA strand)
3) Termination (stop codon recognized)
Promoters
sequences of DNA that signal the initiation of RNA synthesis
i.e. TATA box: rich in A and T and is located about 25nucleotide upstream of the transcription start site
Transcription factors
help eukaryotic RNA polymerase recognize promoter sequences and start transcription
Initiation in transcription
Promoters (TATA box)
Transcription factors
During Elongation
in transcription
RNA polymerase moves along the DNA unraveling the double helix, for pairing with RNA nucleotides
Termination mechanisms
in transcription
require the removal of RNA polymerase from the DNA and is different in prokaryotes and eukaryotes
Enzymes in eukaryotic cells…
modify the mRNA in the nucleus after transcription for its dispatch to the cytoplasm
(proof-read) ?
Translation
3 stages
1) Initiation
2) Elongation
3) Termination
into language of amino acids and polypeptides
a cell translates or interprets…
in translation
a series of codons of an mRNA message into protein with the help of transfer RNA (tRNA)
tRNA transfers
in translation
amino acids from eh cytoplasmic pool to a ribosome and the ribosome adds (catalyzes) each amino acid carried by tRNA to the growing end of the polypeptide chain
Molecules of tRNA are not all identical
each carrie a specific amino acid on one end and has an anticodon on the other end to recognize the mRNA
tRNA pairs…
with the codons of the mRNA to ensure the correct polypeptide is synthesized
structure of tRNA
consists of a single RNA strand that is only about 80nucleotides long and is roughly L-shaped
aminoacyl - tRNA synthetase
joins each amino acid to the correct tRNA, in an ATP-dependent manner
synthesizes the bond between an amino acid and tRNA
Ribosomes facilitate
the specific coupling of tRNA anticodons with mRNA codons during protein synthesis
binds tRNA and facilitates protein synthesis
ribosomal RNA or rRNA
are RNA molecules, along with proteins, that construct the ribosomal subunits
Initiation in translation
brings together mRNA, tRNA bearing the first amino acid of the polypeptide, and two subunits of a ribosome
Elongation in translation
amino acids are added one by one to the preceding amino acid
Termination in translation
the final stage of translation and occurs when the ribosome reaches a stop codon in the mRNA
Polyribosome
formed when a number of ribosomes can translate a single mRNA molecule simultaneously
help with speed and quantity
A single protein can be translated
in less than 1 minute
Completing and targeting the functional protein
polypeptide chains undergo further modifications after the translation process (post-translation modification) that affects their shape, function, or destination in the cell
proteins destined for the endomembrane system or secretion must be transported and synthesized directly into the ER
these proteins have signal peptides enabling the translating ribosome to bind to the ER and synthesize into the ER center
i.e. insulin
important words: 1) post-translation modification; 2) endomembrane system; 3) signal peptides
SRP: signal recognition protein after signal peptide: brings ribosome to ER
Mutations
in nucleic acids can affect protein structure and function
changes in the genetic material of the cell
can occur during DNA or RNA replication, recombination, or repair
Point Mutations
changes in one base pair of a gene that leads to the production of an abnormal protein
i.e. sickle cell disease
change from CTT (Glu) => CAT (Val)
2 types of point mutations
1) Base-pair substitutions
2) Base-pair insertions or deletions
Base-pair substitutions
replacement of one nucleotide and its partner with another pair of nucleotides
Base-pair insertions or deletions
additions or losses of nucleotides pairs in a gene and may produce frameshift mutations
Mutagens
physical or chemical agents that can cause mutations
i.e. UV, oxygen radicals, alkylating agents
Genetic Code
and its associated machinery is nearly universal
is shared by organisms from the simplest bacteria to the most complex animals
in laboratory experiments, genes can be
transplanted between organisms or species
then these genes can be transcribed and translated to make the same proteins
i.e. tobacco plant expressing firefly genes causes the plant to “glow”
