From Genes to Proteins Chapter 17

From Genes to Proteins Chapter 17
AP Chapter 17

How was the fundamental relationship between genes and proteins discovered?
In 1909, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions

He studied alkaptonuria.
Alkaptonuria is best known for the darkening of urine from yellow to brown to black after it is exposed to the air. Garrod studied the patterns in several families, realized it followed an autosomal recessive pattern of inheritance, and postulated that it was caused by a mutation in a gene encoding an enzyme involved in the metabolism of a class of compounds called alkaptans.

Nutritional Mutants in Neurospora: Scientific Inquiry
George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal medium as a result of inability to synthesize certain molecules. They identified mutants that lacked certain enzymes necessary for synthesizing. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The mutants could not produce the enzymes to move through the pathway.
Fig. 17-2 EXPERIMENT The mutants could not produce the enzymes to move through the pathway. Growth: Wild-type cells growing and dividing No growth: Mutant cells cannot grow and divide Minimal medium RESULTS Classes of Neurospora crassa Wild type Class I mutants Class II mutants Class III mutants Minimal medium (MM) (control) MM + ornithine Condition MM + citrulline MM + arginine (control) Figure 17.2 Do individual genes specify the enzymes that function in a biochemical pathway? CONCLUSION Class I mutants (mutation in gene A) Class II mutants (mutation in gene B) Class III mutants (mutation in gene C) Wild type Precursor Precursor Precursor Precursor Gene A Enzyme A Enzyme A Enzyme A Enzyme A Ornithine Ornithine Ornithine Ornithine Gene B Enzyme B Enzyme B Enzyme B Enzyme B Citrulline Citrulline Citrulline Citrulline Gene C Enzyme C Enzyme C Enzyme C Enzyme C Arginine Arginine Arginine Arginine

They developed a one gene–one enzyme hypothesis, which states that each gene dictates production of a specific enzyme

Revision of the hypothesis…
Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein Therefore, Beadle and Tatum’s hypothesis is now restated as the one gene–one polypeptide hypothesis Note that it is common to refer to gene products as proteins rather than polypeptides.

Remember! Proteins are made of amino acids.
RNA is a key-player in Protein Synthesis.

RNA differs from DNA in three major ways.
RNA has a ribose sugar. RNA has uracil instead of thymine. RNA is a single-stranded structure.

URACIL

Different sugars in DNA and RNA
ribose deoxyribose Has one Less oxygen atom

Gene 2 Gene 1 Gene 3 DNA template strand mRNA Codon TRANSLATION
Fig. 17-4 Gene 2 DNA molecule Gene 1 Gene 3 DNA template strand TRANSCRIPTION Figure 17.4 The triplet code mRNA Codon TRANSLATION Protein Amino acid

Basic Principles of Transcription and Translation
Transcription is the synthesis of mRNA (messenger RNA) under the direction of DNA Occurs in the nucleus of eukaryotes Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA Ribosomes are the sites of translation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

DNA TRANSCRIPTION mRNA (a) Bacterial cell Fig. 17-3a-1
Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information (a) Bacterial cell

DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide
Fig. 17-3a-2 DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information (a) Bacterial cell

Differences in Euk and Prok
1. In a eukaryotic cell, the nuclear envelope separates transcription from translation 2. Eukaryotic RNA transcripts are modified through RNA processing to yield finished mRNA. A primary transcript is the initial RNA transcript from any gene. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 17-3 DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide (a) Bacterial cell Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information mRNA TRANSLATION Ribosome Polypeptide (b) Eukaryotic cell

The central dogma is the concept that cells are governed by a cellular chain of command: DNA RNA protein.

The Genetic Code How are the instructions for assembling amino acids into proteins encoded into DNA? There are 20 amino acids, but there are only four nucleotide bases in DNA How many bases correspond to an amino acid? 3

The Genetic Code is read in CODONS which are Triplets of Nucleotide Bases in mRNA that code for amino acids. Example: AGT codes for the amino acid serine. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Cracking the Code All 64 codons were deciphered by the mid-1960s by Marshall Nirenberg Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation There is one “start” code AUG which also codes for the amino acid methionine. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

First mRNA base (5 end of codon) Third mRNA base (3 end of codon)
Fig. 17-5 Second mRNA base First mRNA base (5 end of codon) Third mRNA base (3 end of codon) Figure 17.5 The dictionary of the genetic code

What does this mean? The genetic code is redundant: an amino acid may have more than one code. but not ambiguous; no codon specifies more than one amino acid

Evolution of the Genetic Code
The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals Genes can be transcribed and translated after being transplanted from one species to another Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

(a) Tobacco plant expressing a firefly gene (b) Pig expressing a
Fig. 17-6 Figure 17.6 Expression of genes from different species (a) Tobacco plant expressing a firefly gene (b) Pig expressing a jellyfish gene

In a nutshell…. During transcription, one of the two DNA strands called the template strand provides a template for ordering the sequence of nucleotides in a mRNA. Codons along an mRNA molecule are read by translation machinery in the 5 to 3 direction Each codon specifies the addition of one of 20 amino acids

DNA: TAC AAA TGA GGA TCA GCT ACC CCA ACA ACT
mRNA: AUG UUU ACU CCU AGU CGA UGG GGU UGU UGA Amino acids Met(start)–phe–thr–pro–ser-arg-trp-gly-cys-stop

Transcription is the DNA-directed synthesis of RNA: a closer look
The three stages of transcription: Initiation Elongation Termination Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides. RNA synthesis follows the same base-pairing rules as DNA, except uracil substitutes for thymine Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The DNA sequence where RNA polymerase attaches is called the promoter.
Fig. 17-7a-1 Promoter Transcription unit 5 3 3 5 DNA Start point RNA polymerase The DNA sequence where RNA polymerase attaches is called the promoter. The stretch of DNA that is transcribed is called a transcription unit Figure 17.7 The stages of transcription: initiation, elongation, and termination

Nontemplate Elongation strand of DNA RNA nucleotides RNA polymerase 3
Fig. 17-7b Elongation Nontemplate strand of DNA Promoters (TATA box area) signal the initiation of RNA synthesis. RNA nucleotides RNA polymerase Transcription factors mediate the binding of RNA polymerase and the initiation of transcription 3 3 end 5 Figure 17.7 The stages of transcription: initiation, elongation, and termination 5 Direction of transcription (“downstream”) Adds at 3’ of growing end Template strand of DNA Newly made RNA

Several transcription factors must bind to the DNA before RNA
Fig. 17-8 1 A eukaryotic promoter includes a TATA box Promoter Template 5 3 3 5 TATA box Start point Template DNA strand 2 Several transcription factors must bind to the DNA before RNA polymerase II can do so. Transcription factors 5 3 3 5 3 Additional transcription factors bind to the DNA along with RNA polymerase II, forming the transcription initiation complex. Figure 17.8 The initiation of transcription at a eukaryotic promoter RNA polymerase II Transcription factors Transcription factors 5 3 3 5 5 RNA transcript Transcription initiation complex

Promoter Transcription unit 5 3 3 5 DNA Start point RNA polymerase
Fig. 17-7a-2 Promoter Transcription unit 5 3 3 5 DNA Start point RNA polymerase 1 Initiation 5 3 3 5 RNA transcript Template strand of DNA Unwound DNA Figure 17.7 The stages of transcription: initiation, elongation, and termination

Promoter Transcription unit 5 3 3 5 DNA Start point RNA polymerase
Fig. 17-7a-3 Promoter Transcription unit 5 3 3 5 DNA Start point RNA polymerase 1 Initiation 5 3 3 5 RNA transcript Template strand of DNA Unwound DNA 2 Elongation Rewound DNA 5 3 3 3 5 5 Figure 17.7 The stages of transcription: initiation, elongation, and termination RNA transcript

Fig. 17-7a-4 Promoter Transcription unit 5 3 3 5 DNA Start point RNA polymerase 1 Initiation 5 3 3 5 RNA transcript Template strand of DNA Unwound DNA 2 Elongation Rewound DNA 5 3 3 3 5 5 Figure 17.7 The stages of transcription: initiation, elongation, and termination RNA transcript 3 Termination 5 3 3 5 5 3 animations Completed RNA transcript

A gene can be transcribed simultaneously by several RNA polymerases
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Differences in transcription in Prokaryotes and Eukaryotes
Prok lack transcription factors No terminator sequence in Euk More types of RNA polymerases in euk No RNA processing in Prok

Compare DNA polymerase and RNA polymerase
How they function: DNA polymerase used to bind DNA nucleotides to present DNA RNA polymerase used to bind RNA nucleotides to DNA Requirement for a template and primer: RNA polymerase does not need a primer. Both use DNA as a template.

Direction of synthesis: 5’ to 3’ Type of nucleotides used: DNA-A,T,C,G; RNA – A,U,C,G (U instead of T)

What is a promoter? It is a DNA sequence where transcription will begin and where the RNA polymerase attaches. It is located upstream of a transcription unit.

What makes RNA polymerase start transcribing a gene at the right place on the DNA in a bacterial cell? In an eukaryotic cell? A promoter area In an eukaryotic cell? A promoter area and transcription factors

Eukaryotic cells modify RNA after transcription
Each end of a pre-mRNA molecule is modified in a particular way: The 5 end receives a modified nucleotide 5 cap The 3 end gets a poly-A tail

(UTR – untranslated region)
Fig. 17-9 G Protein-coding segment Polyadenylation signal 5 3 G P P P AAUAAA AAA … AAA 5 Cap 5 UTR Start codon Stop codon 3 UTR Poly-A tail Figure 17.9 RNA processing: addition of the 5 cap and poly-A tail (UTR – untranslated region)

These modifications share several functions:
They facilitate the export of mRNA out of the nucleus They protect mRNA from hydrolytic enzymes They help ribosomes attach to the 5 end

Split Genes and RNA Splicing
Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions called introns. Coding regions are exons RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence

exons spliced together Coding segment
Fig 5 Exon Intron Exon Intron Exon 3 Pre-mRNA 5 Cap Poly-A tail 1 30 31 104 105 146 Introns cut out and exons spliced together Coding segment mRNA 5 Cap Poly-A tail 1 146 Figure RNA processing: RNA splicing 5 UTR 3 UTR

Remove the introns: IJOILKJLOVEIUIOAPJKLBIOLOGYIOY IJOILKJLOVEIUIOAPJKLBIOLOGYI Remember the exons…..exit or go on to make the proteins!

How are the pieces cut and spliced?
Spliceosomes consist of a variety of proteins and small nuclear RNA (snRNA), “snRNP’s”, that recognize the splice sites Did you call me? Oh, “snurps” not smurfs!

RNA transcript (pre-mRNA) 5 Exon 1 Intron Exon 2
Fig RNA transcript (pre-mRNA) 5 Exon 1 Intron Exon 2 Protein Other proteins snRNA snRNPs Figure The roles of snRNPs and spliceosomes in pre-mRNA splicing

Fig RNA transcript (pre-mRNA) 5 Exon 1 Intron Exon 2 Protein Other proteins snRNA snRNPs Spliceosome 5 Figure The roles of snRNPs and spliceosomes in pre-mRNA splicing

Fig RNA transcript (pre-mRNA) 5 Exon 1 Intron Exon 2 Protein Other proteins snRNA snRNPs Spliceosome 5 Figure The roles of snRNPs and spliceosomes in pre-mRNA splicing Spliceosome components Cut-out intron mRNA 5 Exon 1 Exon 2

Ribozymes are catalytic RNA molecules that function as enzymes and can also splice RNA
The discovery of ribozymes rendered obsolete the belief that all biological catalysts were proteins.

The Functional and Evolutionary Importance of Introns
Alternative RNA splicing can enable some genes to encode more than one kind of protein depending on which segments are treated as exons. Introns allow more room for crossing-over which results in exon shuffling may result in the evolution of new proteins

A Closer Look at Translation

Remember… Translation is using the mRNA code to order the amino acids in a polypeptide (protein).

How do the amino acids get to the ribosome?
t-RNA A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a cloverleaf

Molecules of tRNA are not identical:
Fig 3 Amino acid attachment site Molecules of tRNA are not identical: Each carries a specific amino acid on one end. Each has an anticodon on the other end. The anticodon base-pairs with a a complementary codon on mRNA 5 Hydrogen bonds Anticodon (a) Two-dimensional structure 5 Amino acid attachment site 3 Figure The structure of transfer RNA (tRNA) Hydrogen bonds 3 5 Anticodon Anticodon (c) Symbol used in this book (b) Three-dimensional structure

Accurate translation requires two steps:
First: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase Second: a correct match between the tRNA anticodon and an mRNA codon Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon.

Aminoacyl-tRNA Amino acid synthetase (enzyme) Fig. 17-15-1
P P P Adenosine ATP Figure An aminoacyl-tRNA synthetase joining a specific amino acid to a tRNA

Aminoacyl-tRNA Amino acid synthetase (enzyme) Fig. 17-15-2
P P P Adenosine ATP P Adenosine P P i P i P i Figure An aminoacyl-tRNA synthetase joining a specific amino acid to a tRNA

Aminoacyl-tRNA Amino acid synthetase (enzyme) tRNA Aminoacyl-tRNA
Fig Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP P Adenosine tRNA P P i Aminoacyl-tRNA synthetase P i P i tRNA Figure An aminoacyl-tRNA synthetase joining a specific amino acid to a tRNA P Adenosine AMP Computer model

Aminoacyl-tRNA Amino acid synthetase (enzyme) tRNA Aminoacyl-tRNA
Fig Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP P Adenosine tRNA P P i Aminoacyl-tRNA synthetase P i P i tRNA Figure An aminoacyl-tRNA synthetase joining a specific amino acid to a tRNA P Adenosine AMP Computer model Aminoacyl-tRNA (“charged tRNA”)

I’m really worried about my aminoacyl tRNA synthetases…

Ribosomes Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

A ribosome has three binding sites for tRNA:
The P site holds the tRNA that carries the growing polypeptide chain The A site holds the tRNA that carries the next amino acid to be added to the chain The E site is the exit site, where discharged tRNAs leave the ribosome Remember: P first, then A, then E!

(a) Computer model of functioning ribosome
Fig a Growing polypeptide Exit tunnel tRNA molecules Large subunit E P A Small subunit Figure The anatomy of a functioning ribosome 5 3 mRNA (a) Computer model of functioning ribosome

(b) Schematic model showing binding sites
Fig b P site (Peptidyl-tRNA binding site) A site (Aminoacyl- tRNA binding site) E site (Exit site) E P A Large subunit mRNA binding site Small subunit (b) Schematic model showing binding sites Growing polypeptide Amino end Next amino acid to be added to polypeptide chain Figure The anatomy of a functioning ribosome E tRNA mRNA 3 Codons 5 (c) Schematic model with mRNA and tRNA

Fig Codons along an mRNA molecule are read by translation machinery in the 5 to 3 direction Amino acids Polypeptide tRNA with amino acid attached Ribosome Trp Phe Gly Figure Translation: the basic concept tRNA Anticodon 5 Codons 3 mRNA

Building a Polypeptide
The three stages of translation: Initiation Elongation Termination All three stages require protein “factors” that aid in the translation process Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Steps: Initiation First, a small ribosomal subunit binds with mRNA and a special initiator tRNA Then the small subunit moves along the mRNA until it reaches the start codon (AUG met) at the P site Proteins called initiation factors bring in the large subunit that completes the translation initiation complex Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Translation initiation complex
Fig Large ribosomal subunit 3 U C 5 A P site Met 5 A Met U G 3 Initiator tRNA GTP GDP E A mRNA 5 5 3 3 Start codon Figure The initiation of translation Small ribosomal subunit mRNA binding site Translation initiation complex

Elongation of the Polypeptide Chain
tRNA bearing the next amino acid comes in at the A site by codon recognition Amino acids are joined by peptide bonds tRNA that was in the P site moves over into the E site. Everything shifts over. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

More realistic images

Amino end of polypeptide E 3 mRNA 5 Fig. 17-18-1 P A site site
Figure The elongation cycle of translation

GDP Amino end of polypeptide E 3 mRNA 5 E P A Fig. 17-18-2 P A site
GTP GDP E P A Figure The elongation cycle of translation

GDP Amino end of polypeptide E 3 mRNA 5 E P A E P A Fig. 17-18-3 P A
site A site 5 GTP GDP E P A Figure The elongation cycle of translation E P A

http://www.dnalc.org/resources/3d/16-translation-advanced.html GDP GDP
Fig Amino end of polypeptide E 3 mRNA Ribosome ready for next aminoacyl tRNA P site A site 5 GTP GDP E E P A P A Figure The elongation cycle of translation GDP GTP E P A

Termination of Translation
Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome The A site accepts a protein called a release factor The release factor causes the addition of a water molecule instead of an amino acid This reaction releases the polypeptide, and the translation assembly then comes apart

Release factor 3 5 Stop codon (UAG, UAA, or UGA) Fig. 17-19-1
Figure The termination of translation

Release factor Free polypeptide 3 3 2 5 5 Stop codon
Fig Release factor Free polypeptide 3 3 2 5 5 GTP Stop codon (UAG, UAA, or UGA) 2 GDP Figure The termination of translation

Release factor Free polypeptide H2O 5 3 3 3 2 5 5 Stop codon
Fig Release factor Free polypeptide H2O 5 3 3 3 2 5 5 GTP Stop codon (UAG, UAA, or UGA) 2 GDP Figure The termination of translation

Polyribosomes A number of ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome) Polyribosomes enable a cell to make many copies of a polypeptide very quickly Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Completed polypeptide Growing polypeptides Incoming ribosomal subunits
Fig Completed polypeptide Growing polypeptides Incoming ribosomal subunits Polyribosome Start of mRNA (5 end) End of mRNA (3 end) (a) Ribosomes Figure Polyribosomes mRNA (b) 0.1 µm

Protein Folding and Post-Translational Modifications
During and after synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape It may have sugars and lipids bonded to it.

Targeting Polypeptides to Specific Locations
Some will be free proteins and some will be bound to the ER Polypeptides destined for the ER or for secretion are marked by a signal peptide

A signal-recognition particle (SRP) binds to the signal peptide
The SRP brings the signal peptide and its ribosome to the ER

Let’s do this! DNA: (in nucleus)
TAC AAA TGA GGA TCA GCT ACC CCA ACA ACT RNA: (in cytoplasm, at ribosome)

Point mutations can affect protein structure and function
Mutations are changes in the genetic material of a cell or virus Mutagens are physical or chemical agents that can cause mutations Point mutations are chemical changes in just one base pair of a gene

Wild-type hemoglobin DNA Mutant hemoglobin DNA 3 C T T 5 3 C A T 5
Fig Wild-type hemoglobin DNA Mutant hemoglobin DNA 3 C T T 5 3 C A T 5 5 G A A 3 5 G T A 3 mRNA mRNA 5 G A A 3 5 G U A 3 Figure The molecular basis of sickle-cell disease: a point mutation Normal hemoglobin Sickle-cell hemoglobin Glu Val

Types of Point Mutations
Point mutations within a gene can be divided into two general categories Base-pair substitutions Base-pair insertions or deletions Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Substitutions A base-pair substitution replaces one nucleotide and its partner with another pair of nucleotides Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the code Missense mutations still code for an amino acid, but not necessarily the right amino acid Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein

Insertions and Deletions
Insertions and deletions are additions or losses of nucleotide pairs in a gene These mutations have a disastrous effect on the resulting protein more often than substitutions do Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Silent (no effect on amino acid sequence)
Fig a Wild type DNA template strand 3 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end A instead of G 3 5 5 3 Figure Types of point mutations U instead of C 5 3 Stop Silent (no effect on amino acid sequence)

Wild type DNA template strand 3 5 5 3 mRNA 5 3 Protein Stop
Fig b Wild type DNA template strand 3 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end T instead of C 3 5 5 3 Figure Types of point mutations A instead of G 5 3 Stop Missense

Fig c Wild type DNA template strand 3 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end A instead of T 3 5 5 3 Figure Types of point mutations U instead of A 5 3 Stop Nonsense

Frameshift causing immediate nonsense (1 base-pair insertion)
Fig d Wild type DNA template strand 3 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end Extra A 3 5 5 3 Figure Types of point mutations Extra U 5 3 Stop Frameshift causing immediate nonsense (1 base-pair insertion)

Frameshift causing extensive missense (1 base-pair deletion)
Fig e Wild type DNA template strand 3 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end missing 3 5 5 3 Figure Types of point mutations missing 5 3 Frameshift causing extensive missense (1 base-pair deletion)

Fig f Wild type DNA template strand 3 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end missing 3 5 5 3 Figure Types of point mutations missing 5 3 Stop No frameshift, but one amino acid missing (3 base-pair deletion)

Wild-type hemoglobin DNA Mutant hemoglobin DNA 3 C T T 5 3 C A T 5
Fig Wild-type hemoglobin DNA Mutant hemoglobin DNA 3 C T T 5 3 C A T 5 5 G A A 3 5 G T A 3 mRNA mRNA 5 G A A 3 5 G U A 3 POLAR NONPOLAR Figure The molecular basis of sickle-cell disease: a point mutation Normal hemoglobin Sickle-cell hemoglobin Glu Val

What Is a Gene? Revisiting the Question
The idea of the gene itself is a unifying concept of life We have considered a gene as: A discrete unit of inheritance A region of specific nucleotide sequence in a chromosome A DNA sequence that codes for a specific polypeptide chain

In summary, a gene can be defined as a region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule

Fig. 17-UN8

From Genes to Proteins Chapter 17

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