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Molecular Genetics

In eukaryotes, chromosomes bear the genetic information that is passed from parents to offspring.

The genetic information is stored in molecules of DNA. The DNA, in turn, codes for enzymes, which, in turn, regulate chemical reactions that direct metabolism for cell development, growth, and maintenance.

The underlying molecular mechanisms that interpret the informationin DNA to generate these effects is the subject of this section.

The structure of DNA and RNA was presented earlier in the chapter on chemistry. As a review, both DNA and RNA are polymers of nucleotides .The nucleotide monomer consists of three parts:

-a nitrogen base - a sugar

- a phosphate.

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Cloverleafالبرسيم ورقة شكل

The Differences in the Structures of DNA and RNA and Summarizes Their Functions.

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Thymine instead of Uracil

Uracil instead of Thymine

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DNA Replication

During interphase of the cell cycle, a second chromatid containing a copy of the DNA molecule is assembled.

(RNA) (DNA)

a. A chromosome prior to replication contains a single DNA molecule;b. A chromosome that has been replicated and consists of two chromatids,each comprising a single DNA double helix molecule.

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DNA Replication

This process, called DNA replication, involves separating (“unzipping”) the DNA molecule into two strands, each of which serves as a template to assemble a new, complementary strand.

The result is two identical double-stranded molecules of DNA.

Because each of these double-stranded molecules of DNA consists of a single strand of old DNA (the template strand) and a single strand of new, replicated DNA (the complementary strand), the process is called: semiconservative replication.

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Semiconservative Replication

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During DNA replication, the enzyme helicase unwinds the DNA helix, forming a Y-shaped replication fork.

Single-stranded DNA binding proteins attach to each strand of the uncoiled DNA to keep them separate.

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As helicase unwinds the DNA, it forces the double-helix in front of it to twist. A group of enzymes, called topoisomerases, break and rejoin the double helix, allowing the twists to unravel and preventing the formation of knots.

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Since a DNA double-helix molecule consists of two opposing DNA strands, the uncoiled DNA consists of a 3' → 5' template strand and a 5' → 3' template strand.

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The enzyme that assembles the new DNA strand, DNA polymerase, moves in the 3' → 5' direction along each template strand. The new (complement) strands grow in the antiparallel, 5' → 3' direction.

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For the 3' → 5' template strand, replication occurs readily as the DNA polymerase follows the replication fork, assembling a 5' → 3' complementary strand. This complementary strand is called the

leading strand.

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For the 5' → 3' template strand, however, the DNA polymerase moves away from the uncoiling replication fork. This is because it can assemble nucleotides only as it travels in the 3' → 5‘ direction. As the helix is uncoiled, DNA polymerase assembles short segments of nucleotides along the template strand in the direction away from the replication fork.

After each complement segment is assembled, the DNA polymerase must return back to the replication fork to begin assembling the next segment.

These short segments of complementary DNA are called Okazaki segments. The Okazaki segments are connected by DNA ligase, producing a single complement strand. Because this complementary strand requires more time to assemble than the leading strand, it is called the lagging strand.

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DNA polymerase can append nucleotides only to an already existing complementary strand.The first nucleotides of the leading strand and each Okazaki fragment are initiated by RNA primase and other proteins.

RNA primase initiates each complementary segment with RNA (not DNA) nucleotides which serve as an RNA primer for DNA polymerase to append succeeding DNA nucleotides. Later, the RNA nucleotides are replaced with appropriate DNA nucleotides.

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The figure illustrates the growth of leading and lagging DNA complements.

In the figure, the RNA primer that initiated the leading strand is not shown because it was replaced with DNA nucleotides earlier in its synthesis.

The Okazaki fragment of the lagging strand, however, still have its RNA primer attached, because a primer must initiate each new fragment.

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1. Helicase unwinds the DNA, producing a replication fork. Single-stranded DNA binding proteins prevent the single strands of DNA from recombining. Topoisomerase removes twists and knots that form in the double-stranded template as a result of the unwinding induced by helicase. (See 1A, 1B, 1C and 1D in Figure).

The details of DNA replication are summarized below. Numbers correspond to events illustrated in Figure below:

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2. RNA primase initiates DNA replication at special nucleotide sequences (called origins of replication) with short segments of RNA nucleotides (called RNA primers) (see 2A and 2B in Figure).

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3. DNA polymerase attaches to the RNA primers and begins elongation, the adding of DNA nucleotides to the complement strand.

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• 4. The leading complementary strand is assembled continuously as the double-helix DNA uncoils.

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5 .The lagging complementary strand is assembled in short Okazaki fragments, which are subsequently joined by DNA ligase.

)see 5A, 5B, and 5C in Figure.(

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6. The RNA primers are replaced by DNA nucleotides.

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• Energy for elongation is provided by two additional phosphates that are attached to each new nucleotide (making a total of three phosphates attached to the nitrogen base).

• Breaking the bonds holding the two extra phosphates provides the chemical energy for the process.

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Mutations

The replication process of DNA is extremely accurate. In bacteria, the DNA polymerase proofreads the pairing process by checking the newly attached nucleotide to confirm that it is correct.

If it is not, the polymerase removes the incorrect nucleotide, backs up, and attaches a new nucleotide. If a mismatch should escape the proofreading ability of the DNA polymerase, other, mismatch repair, enzymes will correct the error. Repair mechanisms occur in eukaryotic cells as well but are not well understood.

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Radiation (such as ultraviolet and x-ray) and various reactive chemicals can cause errors in DNA molecules.

One kind of DNA error occurs when the bases of two adjacent nucleotides in one DNA strand bond to each other rather than make proper pairs with nucleotides in the complementary DNA strand.

A thymine dimer, for example, originates when two adjacent thymine nucleotides in the same strand base-pair with each other instead of with the adenine bases in

• the complementary strand. Such errors can be fixed by excision repair enzymes that splice out the error and use the complementary strand as a pattern, or template, for replacing the excised nucleotides.

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If a DNA error is not repaired, it becomes a mutation. A mutation is any sequence of nucleotides in a DNA molecule that does not exactly match the original DNA molecule from which it was copied. Mutations include:

1. an incorrect nucleotide (substitution),2. a missing nucleotide (deletion), or 3. an additional nucleotide not present in the original DNA molecule

(insertion).

When an insertion mutation occurs, it causes all the subsequent nucleotides to be displaced one position, producing a frameshift mutation.

Radiation or chemicals that cause mutations are called mutagens. Carcinogens are mutagens that activate uncontrolled cell growth (cancer).

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Protein Synthesis

The DNA in chromosomes contains genetic instructions that regulate development, growth, and the metabolic activities of cells. The DNA instructions determine whether a cell will be that of a pea plant, a human, or some other organism, as well as establish specific characteristics of the cell in that organism.

For example, the DNA in a cell may establish that it is a human cell. If, during development, it becomes a cell in the iris of an eye, the DNA will direct other information appropriate for its location in the organism, such as the production of brown, blue, or other pigmentation. DNA controls the cell in this manner because it contains codes for polypeptides.

Many polypeptides are enzymes that regulate chemical reactions, and these chemical reactions influence the resulting characteristics of the cell.

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In the study of heredity, the terms gene and genotype are used to represent the genetic information for a particular trait.

From the molecular viewpoint, traits are the end products of metabolic processes regulated by enzymes.

When this relationship between traits and enzymes was discovered, the gene was defined as the segment of DNA that codes for a particular enzyme (one-gene-one-enzyme hypothesis).

Since many genes code for polypeptides that are not enzymes (such as structural proteins or individual components of enzymes), the gene has been redefined as the DNA segment that codes for a particular polypeptide (one-gene-one- polypeptide hypothesis).

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The process that describes how enzymes and other proteins are made from DNA is called protein synthesis.

There are three steps in protein synthesis:

-Transcription , - RNA Processing ,

- Translation.

In transcription, RNA molecules are created by using the DNA molecule as a template .

After transcription, RNA processing modifies the RNA molecule with deletions and additions .

In translation, the processed RNA molecules are used to assemble amino acids into a polypeptide.

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There are three kinds of RNA molecules produced during transcription, as follows:

1. Messenger RNA (mRNA)

mRNA is a single strand of RNA that provides the template used for sequencing amino acids into a polypeptide.

A triplet group of three adjacent nucleotides on the mRNA, called a codon, codes for one specific amino acid. Since there are 64 possible ways that four nucleotides can be arranged in triplet combinations (4 × 4 × 4 = 64),

there are 64 possible codons. However, there are only 20 amino acids,

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The genetic code, given in the Figure, provides the “decoding” for each codon.

That is, it identifies the amino acid specified by each of the possible 64 codon combinations.

For example, the codon composed of the three nucleotides cytosine-guanine-adenine (CGA) codes for the amino acid arginine.

This can be found in the Figure by aligning the C found in the first column with the G in the center part of the table and the A in the column at the far right.

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2. Transfer RNA (tRNA)

tRNA is a short RNA molecule (consisting of about 80 nucleotides) that is used for transporting amino acids to their proper place on the mRNA template.

Interactions among various parts of the tRNA molecule result in base-pairings between nucleotides, folding the tRNA in such a way that it forms a three-dimensional molecule. (In two dimensions, a tRNA resembles the three leaflets of a clover leaf.)

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The 3' end of the tRNA (ending with cytosine-cytosine-adenine, or C-C-A-3') attaches to an amino acid.

Another portion of the tRNA, specified by a triplet combination of

nucleotides, is the anticodon.

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tRNA bringing amino acid to ribosome

Free amino acids

Ribosome incorporation amino acid into the growing protein chain

Growing Protein chain

mRNA coping DNA in nucleus

mRNA being translated

During translation, the anticodon of the tRNA base pairs with the codon of the mRNA. Exact base-pairing between the third nucleotide of the tRNA anticodon and the third nucleotide of the mRNA codon is often not required.

This “wobble” allows the anticodon of some tRNAs to base-pair with more than one kind of codon. As a result, about 45 different tRNAs base-pair with the 64 different codons .

)look the table in slide 31(

codonAnticodon

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• 3. Ribosomal RNA (rRNA)

• rRNA molecules are the building blocks of ribosomes. The nucleolus is an assemblage of DNA actively being transcribed into rRNA. Within the nucleolus, various proteins imported from the cytoplasm are assembled with rRNA to form large and small ribosome subunits. Together, the two subunits form a ribosome which coordinates the activities of the mRNA and tRNA during translation.

• Each ribosome has a binding site for mRNA and three binding sites for tRNA molecules.– The P site holds the tRNA carrying the growing polypeptide chain.– The A site carries the tRNA with the next amino acid.– The E site exit site (Discharged tRNAs leave the ribosome at this site)

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A) TranscriptionIs the process of making RNA from a DNA template

Transcription contain three steps:

- Initiation,

- Elongation,

-Termination.

The details follow, with numbers corresponding to events:

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1. Initiation

In initiation, the RNA polymerase attaches to promoter regions on the DNA and begins to unzip the DNA into two strands. A promoter region for mRNA transcriptions contains the sequence T-A-T-A (called the TATA box).

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2. Elongation

Elongation occurs as the RNA polymerase unzips the DNA and assembles RNA nucleotides using one strand of the DNA as a template. As in DNA replication, elongation of the RNA molecule occurs in the 5' → 3' direction. In contrast to DNA replication, new nucleotides are RNA nucleotides (rather than DNA nucleotides), and only one DNA strand is transcribed.

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3. TerminationTermination occurs when the RNA polymerase reaches a special sequence of nucleotides that serve as a termination point. In eukaryotes, the termination region often contains the DNA sequence AAAAAAA.

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B) RNA Processing

Before a mRNA molecule leaves the nucleus, it undergoes two kinds of alterations:

The first modification (alteration) adds special nucleotide sequences to both ends of the mRNA (see 5 in the Figure). A modified guanosine triphosphate (GTP) is added to the 5' end to form a 5' cap

(-P-P-PG- 5').

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A GTP molecule is a guanine nucleotide with two additional phosphate groups (in the same way that ATP is an adenine nucleotide with two additional phosphates).

The 5' cap provides stability to the mRNA and a point of attachment for the small subunit of the ribosome.

GTP molecule chemical structure

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To the 3' end of the mRNA, a sequence of 150 to 200 adenine nucleotides is added, producing a poly-A tail (-A-A-A . . . A-A-3').

The tail provides stability and also appears to control the movement of the mRNA across the nuclear envelope. By controlling its transport and subsequent expression, the poly-A tail may serve to regulate gene expression.

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The second alteration of the mRNA occurs when some mRNA segments are removed. A transcribed DNA segment contains two kinds of sequences—exons, which are sequences that express a code for a polypeptide, and introns, intervening sequences that are noncoding.

The original unprocessed mRNA molecule, called heterogenous nuclear RNA, contains both the coding and the noncoding sequences.

(see 4 in the Figure).

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Before the RNA moves to the cytoplasm, small nuclear ribonucleoproteins, or snRNPs, delete out the introns and splice the exons

(see 6 in the Figure).

46 Splicing the exons process

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C) Translation

After transcription, the mRNA, tRNA, and ribosomal subunits are transported across the nuclear envelope and into the cytoplasm.

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In the cytoplasm, amino acids attach to the 3' end of the tRNAs, forming an aminoacyl-tRNA. The reaction requires an enzyme specific to each tRNA (aminoacyl-tRNA synthetase) and the energy from one ATP. The amino acid-tRNA bond that results is a high-energy bond, creating an “activated” amino acid-tRNA complex.

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As in transcription, translation is categorized into three steps

• Initiation,• Elongation, and• Termination.

Energy for translation is provided by several GTP molecules. GTP acts as an energy supplier in the same manner as ATP.

GTP molecule chemical structure

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The details of translation follow, with numbers corresponding to events illustrated in the Figure.

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1 .Initiation begins when the small ribosomal subunit attaches to a special region near the 5'end of the mRNA.

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2. A tRNA (with anticodonUAC) carrying the aminoacid methionine attaches tothe mRNA (at the “start”

codon AUG) with hydrogenbonds.

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3. The large ribosomal subunitattaches to the mRNA,forming a completeribosome with the tRNA

)bearing a methionine(occupying the P site.

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4 .Elongation begins when the next tRNA (bearing an amino acid) binds to the A site of the ribosome. The methionine is removed from the first tRNA and attached to the amino acid on the newly arrived tRNA.

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The Figure showselongation after severaltRNAs have deliveredamino acids. Thegrowing polypeptide isshown at 4.

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5. The first tRNA,which no longer carries an amino acid, is released. After its release, the tRNA canagain bind with its specific amino acid, allowing repeated deliveries to the mRNA during translation.

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6. The remaining methionine tRNA (together with the mRNA to which it is bonded)moves from the A site to the P site.

Now the A site is unoccupied and a new codon is exposed. This isanalogous to the ribosome moving over one codon.

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7. A new tRNA carrying a new amino acid enters the A site. The two amino acids on the tRNA in the P site are transferred to the new amino acid, forming a chain of three amino acids .

59 The Figure shows achain of four amino acids.

Stop codon

mRNAmovement

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8. As in step 5, thetRNA in the P siteis released, andsubsequent stepsare repeated. Aseach new tRNAarrives, thepolypeptide chainis elongated byone new aminoacid, growing insequence andlength as dictatedby the codons onthe mRNA.

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9 .Termination occurs when the ribosome encounters one of the three “stop” codons (see the Figure). At termination, the completed polypeptide, the last tRNA, and the two ribosomal subunits are released. The ribosomal subunits can now attach to the same or another mRNA and repeat the process.

Once the polypeptide is completed, interactions among the amino acids give it its secondary and tertiary structures.

Subsequent processing by the endoplasmic reticulum or a Golgi body may make final modifications before the protein functions as a structural element or as an enzyme.

DNA Organization

In eukaryotes, DNA is packaged with proteins to form a matrix called chromatin.

The DNA is coiled around bundles of eight or nine histone proteins to form DNA-histone complexes called nucleosomes.

Through the electron microscope, the nucleosomes appear like beads on a string.During cell division, DNA iscompactly organized into chromosomes.

During cell division, DNA is compactly organized into chromosomes. In the nondividing cell, the DNA is arranged as either of two types of chromatin, as follows:

1. Euchromatin describes regions where the DNA is loosely bound to nucleosomes. DNA in these regions is actively being transcribed.

2. Heterochromatin represents areas where the nucleosomes are more tightly compacted, and where DNA is inactive. Because of its condensed arrangement, heterochromatin stains darker than euchromatin.

Euchromatin

Heterochromatin

NucleosomeDNA

Histone modefications

Some DNA segments within a DNA molecule are able to move to new locations. These transposable genetic elements, called transposons (or “jumping genes”) can move to a new location on the same chromosome or to a different chromosome.

Some transposons consist only of DNA that codes for an enzyme that enables it to be transported.

Other transposons contain genes that invoke replication of the transposon .

After replication, the new transposon copy is transported to the new location.

Wherever they are inserted, transposons have the effect of a mutation .

They may change the expression of a gene, turn on or turn off its expression, or have no effect at all.

Recombinant DNA

Recombinant DNA is DNA that contains DNA segments or genes from different sources.

DNA transferred • from one part of a DNA molecule to another, • from one chromosome to another chromosome, or • from one organism to another

all constitute recombinant DNA.

The transfer of DNA segments can occur,

• Naturally through viral transduction, bacterial conjugation, or transposons or

• Artificially through recombinant DNA technology

Transduction is the process by which DNA is transferred from one

bacterium to another by a virus.

DNA

Restriction enzymerecognition sequence

Restriction enzymecuts the DNA intofragments

Sticky end

Addition of a DNAfragment fromanother source

Two (or more)fragments stick

together bybase-pairing

DNA ligasepastes the strand

Recombinant DNA molecule

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Recombinant DNA technology uses restriction enzymes to cut up DNA. Restriction enzymes are obtained from bacteria that manufacture these enzymes to combat invading viruses.

Restriction enzymes are very specific, cutting DNA at specific recognition sequences of nucleotides.

The cut across a double-stranded DNA is usually staggered, producing fragmentsthat have one strand of the DNA extending beyond the complementary strand. The unpaired extension is called a sticky end.

1+2+3( Fragments produced by a restriction enzyme are often inserted into a plasmid because plasmids can subsequently be introduced into bacteria by transformation .

This is accomplished by first treating the plasmid with the same restriction enzyme as was used to create the DNA fragment .

The restriction enzyme will cut the plasmid at the same recognition sequences, producing the same sticky ends carried by the fragments .

4( Mixing the fragments with the cut plasmids allows base-pairing at the sticky ends. Application of DNA ligase stabilizes the attachment.

5+6( The recombinant plasmid is then introduced into a bacterium by transformation. By following this procedure, the human gene for insulin has been inserted into E. coli.The transformed E. coli produce insulin which is isolated and used to treat diabetes.

Transformation, the taking up of DNA from the fluidsurrounding the cell

Restriction fragments can be separated by gel electrophoresis .

In this process, DNA fragments of different lengths are separated as they diffuse through a gelatinous material under the influence of an electric field .

Since DNA is negatively charged (because of the phosphate groups), itmoves toward the positive electrode .

Shorter fragments migrate further through the gel than longer, heavier fragments.

Gel electrophoresis is often used to compare DNA fragments ofclosely related species in an effort to determine evolutionary relationships.

Mixture of DNA molecules of different size

Shorter

molecules

Longer

molecules

Completed gel

When restriction fragments between individuals of the same species are compared, the fragments differ in length because of polymorphisms, which are slight differences in DNA sequences.

These fragments are called restriction fragment length polymorphisms, or RFLPs.

In DNA fingerprinting, RFLPs produced from DNA left at a crime scene are compared to RFLPs from the DNA of suspects.

Problem in recombinant DNA

When foreign genes are insertedinto the genome of a bacterium with recombinant DNA technology, introns often prevent their transcription.

Also the presence of introns (non coding sequence) in the gene of interest make this gene too large to be cloned easily.

To avoid this problem, the DNA fragment bearing the required gene is obtained directly from the mRNA that codes for the desired polypeptide .

Reverse transcriptase (obtained fromretroviruses) is used to make a DNA molecule directly from the mRNA. DNA obtained in this manner is calledcomplementary DNA, or cDNA, and lacks the introns

Instead of using a bacterium to clone DNA fragments, some fragments can be copied millions of times by using DNA polymerase directly .

This method, called polymerase chain reaction, or PCR, uses synthetic primers that initiate replication at specific nucleotide sequences.