Discontinuously there would be no dna strand to

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
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 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
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 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
