Chapter 17 Flashcards
gene expression
In other words, proteins are the
link between genotype and phenotype. Gene expression is the process by which
DNA directs the synthesis of proteins (or, in some cases, just RNAs).
one gene one polypep hyp
. For example, hemoglobin—the
oxygen-transporting protein of vertebrate red blood cells—
contains two kinds of polypeptides (see Figure 5.18), and
thus two genes code for this protein, one for each type of
polypeptide. Beadle and Tatum’s idea was therefore restated
as the one gene–one polypeptide hypothesis. E
why is calling it the one gene one polypep hyp not rly correct
Even this description is not entirely accurate, though. First, in many cases, a
eukaryotic gene can code for a set of closely related polypeptides via a process called alternative splicing, which you will
learn about later in this chapter. Second, quite a few genes
code for RNA molecules that have important functions incells even though they are never translated into protein
RNA is chemically similar to DNA except that
t it contains ribose
instead of deoxyribose as its sugar and has the nitrogenous
base uracil rather than thymine
transcription
Transcription is the synthesis of RNA using information
in the DNA. The two nucleic acids are written in different
forms of the same language, and the information is simply
transcribed, or “rewritten,” from DNA to RNA. J
mrna
For a protein-coding gene, the resulting
RNA molecule is a faithful transcript of the gene’s proteinbuilding instructions. This type of RNA molecule is called
messenger RNA (mRNA) because it carries a genetic message from the DNA to the protein-synthesizing machinery of
the cell.
translation
Translation is the synthesis of a polypeptide using the
information in the mRNA. During this stage, there is a change
in language: The cell must translate the nucleotide sequence of a polypep
ribosomes
The sites of translation are ribosomes, molecular complexes that facilitate the orderly linking of amino
acids into polypeptide chains.
triplet code
Experiments have verified that the flow of information from
gene to protein is based on a triplet code: The genetic
instructions for a polypeptide chain are written in the DNA
as a series of nonoverlapping, three-nucleotide words
template strand
This strand is called the
template strand because it provides the pattern, or template, for the sequence of nucleotides in an RNA transcript.
coding strand
These codons are complementary to the template strand and
thus identical in sequence to the mRNA, except that they
have a T wherever there is a U in the mRNA. For this reason,
the nontemplate DNA strand is often called the coding
strand; by convention, the sequence of the coding strand
is used when a gene’s sequence is reported
Because diverse forms of life share a
common genetic code due to their shared ancestry,
one species can be
programmed to produce proteins characteristic of a second species by
introducing DNA from the second species into the first (jellyfish pig)
rna pol
. An enzyme called an
RNA polymerase pries the two strands of DNA apart and
joins together RNA nucleotides complementary to the DNA
template strand, thus elongating the RNA polynucleotide
terminator
; in bacteria, the
sequence that signals the end of transcription is called the
terminator. (The termination mechanism is different in
eukaryotes; we’ll describe it later.)
rna procesing
Enzymes in the eukaryotic nucleus modify pre-mRNA in
specific ways before the genetic message is dispatched to the
cytoplasm. During this RNA processing, both ends of the
primary transcript are altered. Also, in most cases, certain
interior sections of the RNA molecule are cut out and the
remaining parts spliced together. These modifications
produce an mRNA molecule ready for translatio