discontinuously, there would be no DNA strand to provide free 3’-OH o Either the telomere must have a unique structure that facilitates its replication or there must be a special enzyme that resolves this Telomerase: An enzyme that adds telomere sequences to the ends of eukaryotic chromosomes Chapter 11: Transcription and RNA Processing Transfer of Genetic Information: The Central Dogma Transcription and Translation Transcript: The strand of DNA of a gene that is used as a template to synthesis a complementary strand of RNA 17
During translation, the sequence of nucleotides in the RNA transcript is converted into the sequence of amino acids in the polypeptide gene product o Conversion is governed by the genetic code , which is the specification of amino acids by nucleotide triplets called codons Translation takes place on intricate macromolecular machines called ribosomes o Ribosomes are composed of three to five molecules and 50-90 different proteins Messenger RNAs (mRNAs): The RNA molecules that are translated on ribosomes o In prokaryotes, the product of transcription is the primary transcript which is equivalent to the mRNA molecule o In eukaryotes, primary transcripts are precursors to mRNAs and are called pre-mRNAs Introns, are noncoding sequenced in higher eukaryotes and some lower eukaryotes o Separate expressed sequences, Exons o Entire sequenced of these split genes, are transcribed into pre- mRNAs and the introns are removed by spliceosomes Five Type of RNA Molecules Transfer RNAs: Small RNA molecules that function as adaptors between amino acids and codons in mRNA during translation Ribosomal RNAs: Structural and catalytic components of the ribosomes o Intricate machines that translate nucleotide sequenced of mRNAs into amino acid sequences of polypeptides 18
Small nuclear RNAs: Structural components of spliceosomes o Nuclear organelles that excise introns from gene transcripts Mincro RNAs: Short 20-22 nucleotide single-stranded RNAs that are cleaved from small hairpin-shaped precursors o Block expression of complementary mRNAs by causing their degradation or repressing translation Messenger RNAs All five are produced by transcription Final products of tRNA, rRNA, snRNA and miRNA are RNA molecules o These molecules are not translated The Process of Gene Expression General Features of RNA Synthesis RNA synthesis occurs by a mechanism similar to DNA synthesis, except that: o Precursors are ribonucleotide triphosphates o Only one strand of DNA is used as the template o RNA chains can be initiated de novo , without requirement for a preexisting primer strand RNA molecule produced will be complementary and antiparallel to the DNA template strand and identical to the DNA non-template (Excpet for uridine residue instead of thymine) o Template Strand: The DNA strand that is copied. Also, called the antisense or non-coding o Non-Template strand: The non-transcribed strand of DNA. Also, called the sense, or coding strand
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