Chapter 17 (1)
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The information content of genes is in the form of specific sequences of __along strands of __, the genetic material.
The __ inherited by an organism leads to specific traits dictating the synthesis of proteins and of _ molecules involved in __.
· In other words, __ are the link between genotype and phenotype
- protein synthesis
__ is the process by which DNA directs the synthesis of proteins (or in some cases, just RNAs). It includes two stages: __ and __.
- It involves similar processes in all domains of life.
- Gene expression
In 1909, British physician __ was the first to suggest that genes dictate phenotypes through enzymes that catalyze specific chemical reactions in the cell.
· Postulated that the symptoms of an inherited disease reflect a person’s inability to make a particular enzyme.
o Such diseases were called ___
o Used __(urine is black because it contains the chemical alkapton which darkens with air exposure) as an example. People either have the enzyme that metabolizes alkapton or not.
- Archibold Garrod
- “inborn errors of metabolism.”
Garrod hypothesized that a gene dictates the production of a specific enzyme.
· Biochemists found evidence that cells synthesize and degrade most organic molecules via ___, in which each chemical reaction in a sequence is catalyzed by a specific enzyme.
__ and __worked with bread mold, Neurospora crassa.
- Caused genetic changes by using __and searching for mutants differing in nutritional needs.
- Beadle and Edward Tatum
Wild type Neurospora has modest food requirements; it can survive on a moist support called __, mixed only with inorganic salts, glucose, and the vitamin biotin.
o From this __, the mold cells use their __to produce all the other molecules they need.
- minimal medium (MM)
- metabolic pathways
The mutants could not survive on __, because they were unable to synthesize certain essential molecules from the minimal ingredients.
· Beadle and Tatum allowed them to grow on a __, which consisted of minimal medium supplemented with all 20 __and a few other nutrients.
o This medium could support any mutant incapable of synthesizing one of the supplements.
- complete growth medium
- amino acids
Beadle and Tatum then distributed them to vials with __ plus a single additional nutrient.
· The particular supplement that allowed growth indicated the __.
- minimal medium
- metabolic defect.
Because each mutant was defective in a single gene, Beadle and Tatum’s results provided strong support for the __, which states that the function of a gene is to dictate the production of a specific enzyme.
· Further support for this hypothesis came from experiments that identified the specific enzymes lacking in the mutants.
one gene-one enzyme hypothesis
Revisions were made to the one gene-one enzyme hypothesis.
What were they?
o Because proteins that are not enzymes are nevertheless gene products, it was thought about in terms of one gene-one protein.
§ However, many proteins are constructed from two or more different ___, and each polypeptide is specified by its own gene.
It was restated as the ___, which still isn’t exactly correct.
- First, not all proteins are enzymes
- polypeptide chains
- one gene-one polypeptide hypothesis
Why wasn't the one gene-one polypeptide exactly correct?
- First, many eukaryotic genes can code for a set of closely related polypeptides in a process called alternative splicing.
- Second, quite a few genes code for RNA molecules that have important functions in cells even though they are never translated into protein.
__provide the instructions for making specific proteins. But a gene does not build a protein directly.
The bridge between DNA and protein synthesis is __.
Differences between DNA and RNA:
- DNA: sugar deoxyribose, double-stranded, nitrogenous base Thymine
- RNA: sugar ribose, single-stranded, nitrogenous base Uracil
Both nucleic acids and proteins are polymers with specific sequences of monomers that convey information.
In DNA or RNA, the monomers are the four types of __, which differ in their nitrogenous bases.
True or False:
Genes are typically hundreds or thousands of nucleotides long, each gene having a specific sequence of bases.
Each polypeptide of a protein also has monomers arranged in a particular linear order (the protein’s __), but its monomers are __.
· Thus, __ and __contain information written in two different chemical languages.
- primary structure
- amino acids
- nucleic acids and proteins
Getting from DNA to protein requires two major stages: ___
transcription and translation.
__is the synthesis of RNA under the direction of DNA.
· Both __ use the same language, and the information is simply transcribed, or copied, from one molecule to the other.
· __ serves as a template strand for DNA replication, as well as for the synthesis of a complementary sequence of RNA nucleotides.
- nucleic acids
For a protein-coding gene, the resulting RNA molecule is a faithful transcript of the gene’s protein-building instructions, and can be sent out in multiple copies.
·This type of RNA is __ because it carries a genetic message from the DNA to the protein-synthesizing machinery of the cell. (__is used as a general term for the synthesis of any RNA molecule.)
__ is the synthesis of a polypeptide, which occurs under the direction of mRNA.
· During this, there is a change in language: The cell must translate the base sequence of an mRNA molecule into the __of a __.
· The sites of translation are __, complex particles that facilitate the orderly linking of amino acids into polypeptide chains.
- amino acid sequence
__ and __ occur in every organism.
· Organisms in the domain __ and __are grouped as prokaryotes because their cells lack a membrane-bound nucleus.
- Transcription and translation
- Bacteria and Archaea
True or False:
The basic mechanics of transcription and translation are similar for bacteria and eukaryotes, but there is an important difference in the flow of genetic information within the cells.
Because bacteria don’t have __, their DNA is not segregated from ribosomes and the other protein-synthesizing equipment.
o This lack of segregation allows __of an mRNA to begin while its __is still in progress.
In a eukaryotic cell, by contrast, the __ separates transcription from translation in space and time.
o __ occurs in the nucleus, and mRNA is transported to the cytoplasm, where __occurs.
- nuclear envelope
Before they can leave the nucleus, eukaryotic RNA transcripts from protein-coding genes are modified to produce the final, functional __.
§ The transcription of a protein-coding eukaryotic gene results in__, and further processing yields the finished mRNA.
§ The initial RNA transcript from any gene, including those coding for RNA that is not translated into protein, is more generally called a __.
- primary transcript
Genes program protein synthesis via genetic messages in the form mRNA. __·
-This concept is called the __ by Francis Crick in 1956
· Some __ molecules can act as templates for DNA.
- DNA -> RNA -> proteins
- central dogma
There are only __nucleotide bases to specify __amino acids.
__ of nucleotide bases are the smallest units of uniform length that can code for all the amino acids.
If each arrangement of the __ specifies an amino acid, there can be __ (43) possible code words- more than enough to specify all the __.
· The flow of information from gene to protein is based on a __: the genetic instructions for a polypeptide chain are written in the DNA as a series of nonoverlapping, three nucleotide words.
- three consecutive bases
- amino acids
- triplet code
During __, the gene determines the sequence of bases along the length of an mRNA molecule. For each gene, only one of the two DNA strands is transcribed.
· This strand, the __, provides the pattern, or __, for the sequence of nucleotides in an __.
- template strand
- RNA transcript
A given DNA strand is the __ for some genes along a DNA molecule, while for other genes the complementary strand functions as the __.
· For a particular gene, the same strand is used as the template every time it is transcribed.
AN mRNA molecule is __rather than identical to its DNA template because RNA bases are assembled on the template according to __.Like DNA, the RNA molecule is __ to the template strand of DNA.
- base-pairing rules
The mRNA base triplets are called __, and they are customarily written in the 5->3’ direction.
The term __is also used for the DNA base triplets along the __strand.
o These __ are complementary to the template strand and thus identical in sequence to the mRNA except that they have T instead of U. (For this reason, the nontemplate DNA strand is sometimes called the “__.”
- coding strand
During __, the sequence of __along an mRNA molecule is decoded, or translated, into a sequence of amino acids making up a polypeptide chain.
· The __ are read in the 5'->3’ direction along the mRNA.
Each __specifies which one of the __ amino acids will be incorporated at the corresponding position along a polypeptide. Because __are base triplets, the number of nucleotides making up a genetic message must be three times the number of __in the protein product.
The first __was deciphered by Marshall Nirenberg in 1961.
· He synthesized an artificial mRNA by linking identical RNA nucleotides containing __as their base. No matter where the message started/stopped, it could contain only one codon in repetition:__.
· Nirenberg added this “__” to a test-tube mixture with ribosomes, amino acids, etc.
· His artificial system translated the poly-U into a polypeptide containing many units of the amino acid __.
All __codons were deciphered by the mid-1960s.
· __ of the __triplets code for amino acids.
The three codons that do not designate amino acids are “__” signals, or __codons, marking the end of translation.
· Genetic messages begin with the mRNA codon __, which signals the protein-synthesizing machinery to begin translating the mRNA at that location.
There is a redundancy in the genetic code, but no ambiguity.
What does this mean?
Although the amino acids repeat, they are not different.
Our ability to extract the intended message from a written language depends on reading the symbols in the correct groupings-that is, the correct __.
· The short stretch of polypeptide will be made correctly only if the mRNA nucleotides are read from ___ (5->3’)
- reading frame
- left to right
Although a genetic message is written with no spaces between the __, the cell’s protein-synthesizing machinery reads the message as a series of nonoverlapping three-letter words.
The __is nearly universal, shared by organisms from the simplest bacteria to the most complex plants and animals.
Exceptions to the universality of the genetic code include __in which a few codons differ from the standard ones.
Slight variations in the genetic code exist in certain unicellular eukaryotes and in the organelle genes of some species.
· There are also exeptions in which __can be translated into one of two amino acids not found in most organisms.
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