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Chapter

BIOL 1201 Chapter Notes -Base Pair, Ribosomal Rna, Intron


Department
Biological Sciences
Course Code
BIOL 1201
Professor
All

Page:
of 4
Chapter 17:
The information content of DNA is in the form of specific sequence of nucleotides
DNA inherited by an organism leads to specific traits dictated by protein synthesis
Proteins link genotype and phenotype
Gene expression- process by which DNA directs protein synthesis, includes two stages:
transcription (DNA to mRNA occurs in nucleus) and translation (RNA to protein occurs in
ribosomes)
RNA is intermediate b/w genes and proteins for which they code
Transcription: synthesis of messenger RNA under direction of DNA
Translation: synthesis of protein at ribosome under direction of mRNA
In prokaryotes, mRNA produced by transcription is immediately translated w/o more processing
In eukaryotes, the nuclear envelope separates transcription
-Primary eukaryotic RNA transcripts modified through RNA processing
-Fig. 17-3 in slideshow
20 amino acids but only 4 nucleotide bases in DNA*
3 bases correspond to an amino acid. This is called a codon.
-Ex: AGT at a particular position on a DNA strand results in placement of amino acid serine at
that corresponding position of the polypeptide to be produced
During transcription, one of the two DNA strands called the template strand provides a template
for ordering the sequence of nucleotides in an RNA transcript
During translation, the mRNA base triplets, called codons, are read in the 5’ to 3’ direction
All 64 codons were deciphered by the mid-1960s
Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation
The genetic code is redundant but not ambiguous; each codon stands for only one amino acid;
however, most amino acids are coded for by more than one codon
-START: AUG-Methionine
-STOP: UGA, UAG, UAA (stop codons code for STOP, not for any amino acids)
Genetic code nearly universal
-Genes can be transcribed and translated after being transplanted from one species to another
MOLECULAR COMPONENTS OF TRANSCRIPTION
Transcription is catalyzed by RNA polymerase
RNA synthesis follows the same base-pairgin rules as DNA, except uracil subs for thymine
DNA sequence where RNA polymerase ……………………………
3 STAGES OF TRANSCRIPTION
1. Initiation
2. Elongation
3. Termination
Enzymes in euk nucleus modify pre-mRNA before the genetic messages are dispatched to the
cytoplasm
During RNA processing, both ends of the primary transcript are usually altered
Interior parts cut out and other parts sliced together…………………
Each end of a pre-mRNA molecule is modified:
-5’ end receives a modified nucleotide 5’ cap
-3’ end gets a poly-A tail
These modifications share several functions:
-facilitate export of mRNA
-protect mRNA from hydrolytic enzymes
……………….
Introns: noncoding stretches of nucleotides that lie b/w coding regions in euks
Exons: other regions; they exit the nucleus and are usually translated into amino acid sequences
(part of mRNA that actually gets translated)
RNA slicing: removes introns and joins exons, creating mRNA molecule with a continuous
coding sequence
A cell translates an mRNA message into protein with the help of transfer RNA
Molecules of tRNA not identical: each carries a specific amino acid on one end and an anticodon
on the other end
-the anticodon base-pairs with a complementary codon on mRNA
RIBOSOMES
-facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis
2 ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)
-protein synthesis occurs 24/7
On ribosomes, P site holds however much protein made so far (tRNA that holds polypeptide…)
A-site holds next amino acid to be added to polypeptide chain
E site is exit site, where discharged tRNAs leave ribosome
3 STAGES OF TRANSLATION
1. Initiation- brings together mRNA, a tRNA with the 1st amino acid, and the 2 ribosomal
subunits
-Then small subunit moves along mRNA until reaches start codon (P site)
2. Elongation- amino acids are added one by one to the preceding amino acid
-Each addition involves proteins called elongation factors and occurs in 3 steps: codon
recognition, peptide bond formation, and translocation
3. Termination-occurs when a stop codon in the mRNA reaches the A site of the ribosome
-A-site accepts a protein called a release factor which causes the addition of a water molecule
instead of an amino acid
-This reaction releases polypeptide and translation assembly then comes apart
*All stages require protein factors to help out*
POLYRIBOSOMES
A number of ribosomes can translate a single mRNA simultaneously, forming polyribosome……
During and after synthesis, a polypeptide chain spontaneously coils and folds into its…
Base pair sub: EX: GACT to GAGT