Important words and concepts from Chapter 17,
Campbell & Reece, 2002 (1/29/2005):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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(1) Chapter title: From Gene to Protein
(a)
“The DNA inherited by an organism leads to specific traits by dictating the synthesis of certain proteins. Proteins are the links between genotype and phenotype.”
(b)
[from gene to protein
(Google Search)] [index]
(2) Central dogma
of molecular genetics
(a)
The central dogma of molecular genetics is typically depicted as a shorthand
review of how genetic information moves around a cell, or from parent to
offspring.
(b)
The central dogma looks like this:
(c)
Note that we can give names to these various steps:
(i)
DNA à DNA = replication
(direction of arrow is arbitrary)
(ii)
DNA à RNA = transcription (direction of arrow is not
arbitrary)
(iii)
RNA à protein = translation (ditto)
(d)
This chapter deals particularly with the last two, transcription and
translation
(e)
(f)
(reverse transcription serves as an exception to the central dogma as
originally conceived; it consists of DNA ß RNA, i.e., RNA à DNA, and is employed by
such things as retroviruses including the virus that causes AIDS;
note in the above figure that, of course, proteins also serve as enzymes)
(g)
[central dogma, central dogma molecular
genetics (Google Search)] [index]
(3)
RNA (uracil)
(a)
See Figure 17.2, Overview:
the roles of transcription and translation in the flow of genetic information
(b)
RNA is a nucleic acid polymer that resembles DNA except
(i)
RNA uses the sugar ribose instead of deoxyribose
·
Ribose has an –OH group at the 2’ carbon instead of the –H seen with
deoxyribose found in DNA
(ii)
RNA employs the nitrogenous base
uracil (U) instead of the pyrimidine thymine
·
For the latter point, that is, T » U
·
The analogous base-pairing is U-A
·
Note that U is energetically cheaper to make than T but that U is also
less stable than T
(c)
[RNA, uracil (Google Search)] [index]
(4) One gene-one
polypeptide hypothesis
(a)
Beadle and Tatum developed the one
gene-one enzyme hypothesis in the 1940s
(b)
The idea is that Mendel’s hereditary units are found in DNA but work by
specifying enzymes
(c)
This hypothesis was modified to one
gene-one protein
since not all proteins are enzymes but genes work by
specifying proteins
(d)
Finally, this hypothesis was modified to one gene-one polypeptide since many proteins consist of more than
one polypeptide
(e)
Genes specify the construction of specific polypeptides
(f)
(in fact, to deconstruct things further, genes specify the transcription of specific RNAs)
(g)
See Figure 17.1, Beadle and
Tatum’s evidence for the one gene—one enzyme hypothesis
(h)
["one gene one
protein", "one gene one
polypeptide", "one gene one
peptide" (Google Search)] [index]
TRANSCRIPTION
(5) Transcription—introduction (template strand)
(a)
The DNA à RNA
flow of genetic information is termed transcription
(b)
The term transcription reflects that the information in DNA (i.e., nucleotide sequence) is copied into a similar code in RNA
(c)
Only one strand of the two possible strands of DNA is typically copied (always one strand per transcriptional
unit)
(d)
In different places on a chromosome the
other strand may be copied
(e)
The DNA strand that provides the complementary template to RNA
polymerization is called the template
strand
(f)
The RNA can be of a number of types including:
(i)
Messenger RNA (mRNA)
(ii)
Transfer RNA (tRNA)
(iii)
Ribosomal RNA (rRNA)
(iv)
Etc. (e.g., spliceosomes)
(g)
See Figure 17.2, Overview:
the roles of transcription and translation in the flow of genetic information
(h)
(see transcription in detail, below)
(i)
[RNA transcription, template strand (Google Search)] [index]
(a)
If the RNA produced by transcription is to
be used to code for the synthesis of proteins,
it is called messenger RNA (a.k.a., mRNA)
(b)
[messenger RNA, mRNA (Google Search)] [index]
(a)
Another category of RNA, used during protein synthesis to ferry amino acids to
growing peptide chains, is called transfer RNA (a.k.a., tRNA)
(b)
(for more information, see transfer RNA,
below)
(c)
[transfer RNA, tRNA (Google Search)] [index]
(a)
Another category of RNA that together constitute about 60% of the mass
of ribosomes is called ribosomal RNA or rRNA
(b)
(in Escherichia coli cells,
ribosomes make up 25% of the dry weight of cells)
(c)
[ribosomal RNA, rRNA (Google Search)] [index]
(a)
The RNA à protein flow of genetic information is termed
translation
(b)
The term translation reflects that the information in mRNAs (i.e., nucleotide sequence) is translated into a new “language”,
i.e., amino acid sequence
(c)
See Figure 17.2, Overview:
the roles of transcription and translation in the flow of genetic information
(d)
(see translation in detail, below)
(e)
[protein translation
(Google Search)] [index]
(10) Eucaryotic segregation of transcription and translation
(a)
Note that due to the existence of the nuclear membrane in eucaryotes, there exists a temporal and spatial separation of transcription and translation
(b)
See Figure 17.2, Overview:
the roles of transcription and translation in the flow of genetic information
(c)
Transcription occurs within the nucleus, where the DNA
resides
(d)
Translation occurs within the cytosol, where the
functional ribosomes reside
(e)
There is no such segregation of transcription and translation in
prokaryotes
(f)
[segregation of translation and
transcription (Google Search)] [index]
(a)
The DNA and RNA nucleotide sequence
code consists of one of four types of nucleotides (4 each, that is)
(b)
The amino acid sequence code consists of 20
amino acids
(c)
In translating from nucleotide sequence to amino acid sequence there
cannot be a one-to-one correspondence (4 < 20)
(d)
There also cannot be a two-to-one correspondence (42 <
20)
(e)
Instead there exists a three to one correspondence (43 >
20)
(f)
The three nucleotides that specify an amino acid during translation are
called codons
(g)
See Figure 17.3, The triplet
code
(h)
See Figure 17.4, The
dictionary of the genetic code
(i)
[codons or codon (Google Search)] [the genetic code
(the table of codons and what that means) (Shaun D. Black)] [index]
(12) Codons are a
property of mRNA
(a)
Note that codons exist in mRNA, but only
their complement exists on the template strand of DNA
(b)
(though note, additionally, that on the non-template strand of DNA the
analogous DNA codons—though without uracil—exists)
(c)
See Figure 17.4, The
dictionary of the genetic code
(d)
[codons mRNA (Google Search)] [index]
(13) Redundancy of
triplet code
(a)
43 = 64 >> 20
(b)
Consequently, there are many more codons than there
are amino acids
(c)
However, 61 of the 64 possible codons do code for an amino acid
(d)
This is because many amino acids are specified by more than one codon
(e)
See Figure 17.4, The
dictionary of the genetic code
(f)
(no, you don’t have to memorize the figure)
(g)
[triplet code redundancy OR
redundant (Google Search)] [index]
(14) Lack of
ambiguity in the triplet code
(a)
Note that while the code is redundant, it is not ambiguous
(b)
That is, each codon
specifies for one and only one amino acid,
not more than one
(c)
[triplet code ambiguity
(Google Search)] [index]
(a)
Another property of codons is that they are arrayed one after
another in the mRNA
(b)
That is, they do not overlap
(c)
(note that there is an only slightly related exception in which codons
can overlap and this is when reading frames of different genes overlap)
(d)
See Figure 17.3, The triplet
code
(e)
[codons overlap (Google Search)] [index]
(16) There is no punctuation between codons
(a)
Furthermore, codons do not have gaps between them (i.e.,
there is no punctuation)
(b)
See Figure 17.3, The triplet
code
(c)
[punctuation codons
(Google Search)] [index]
(17)
Start codon (AUG, methionine)
(a)
The codon AUG codes for the amino acid
methionine
(b)
See Figure 17.4, The
dictionary of the genetic code
(c)
AUG also specifies the initiation of translation
(d)
Thus, all polypeptides initially begin with methionine (Met)
(e)
Note that as a part of post-translational protein processing the Met
amino acid is often clipped off
(f)
(though I don’t expect you to learn all of the codons and their
assignments, you should memorize AUG, methionine, and the fact that it serves
as the start codon of reading frames)
(g)
[start codon, methionine (Google Search)] [index]
(a)
Only 61 of the 64 possible codons specify amino acids
(b)
The other three specify what are known as stop codons
(c)
(or nonsense codons to distinguish them from the other 61 sense codons)
(d)
See Figure 17.4, The
dictionary of the genetic code
(e)
Stop codons instruct the ribosome to stop
adding amino acids to the growing peptide chain
(f)
[stop codon (Google Search)] [index]
(a)
The sequence of codons beginning with AUG and ending with a stop
codon is called the reading frame
(b) <