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Botany 1050

Introduction to Botany

Spring 2009


POLYPEPTIDE (PROTEIN) SYNTHESIS

We now know that DNA carries the genetic material and we also know how it replicates. But how is the information stored in the DNA translated to form the molecules and processes that make up life?

I. The nature of genes

A. Garrod (1900) investigated inherited human disease called alkaptonuria

1. In affected individuals urine not completely broken down, toxins accumulate in the body causing a variety of adverse effects, most notably mental retardation

2. Garrod hypothesized that affected individuals lack a key enzyme needed to break down urine

3. First suggestion that genes might be responsible for the production of enzymes

    B. Beadle and Tatum (1940's) - one gene one enzyme hypothesis

      1. Induced nutritional mutants of red bread mold, Neurospora using X-rays

      2. Some resulting cultures lacked a particular enzyme for growth on medium, e.g. some mutants could not synthesize certain vitamins or amino acids necessary for their growth. These had to be added to their diets.

      3. They found that a single gene was mutated, which resulted in the lack of a single enzyme

      4. By analyzing many types of mutants Beadle and Tatum hypothesized:

a. Each step in a biochemical pathway is controlled by a specific enzyme

b. Each step in a biochemical pathway is controlled by a specific gene

c. Therefore, one gene causes one specific enzyme to be formed

      5. Beadle and Tatum, who won a Nobel prize for their work, demonstrated the role of enzymes in biochemical reactions and that genes code for specific enzymes

II. How genes are expressed

A. Gene-product relationship

1. DNA is in cell nucleus yet protein synthesis takes place in cytoplasm

2. RNA (ribonucleic acid) molecules are the intermediaries between DNA and proteins 

B. RNA

1. RNA is a polymer of nucleotides

2. RNA is single-stranded, contains the sugar ribose, and contains the base uracil instead of thymine

3. Three types of RNA:

a. Messenger RNA (mRNA) takes a message from DNA in nucleus to ribosomes in cytoplasm

b. Ribosomal RNA (rRNA) and proteins make up ribosomes where proteins are synthesized

c. Transfer RNA (tRNA) transfers a particular amino acid to a ribosome

4. Recall that RNA differs from DNA in three ways. RNA:

a. is a single strand.

b. has an extra oxygen on the ribose unit.

c. contains uracil rather than thymine.

C. Overview of protein synthesis

1. Transcription - DNA strand serves as a template for the formation of mRNA

2. Translation - mRNA strand is used to make correct sequence of amino acids during protein synthesis

III. How genes code for amino acids 

A. Sequence of bases in DNA

1. Codes for proper sequence of amino acids in a polypeptide

2. The genetic code is a triplet code comprised of 64 three-base code words (codons)

3. Codon - three consecutive nucleotides on a strand of mRNA which code for a particular amino acid

B. Genetic code

1. Code is degenerate; 64 codons code for 20 amino acids

2. One start codon and three stop codons

3. Code is universal, supports theory of evolution

IV. Transcription - the process by which the genetic code in the DNA is read and used to make a strand of mRNA

A. Takes place in the nucleus:

1. Unwinding - just like in DNA replication the strands separate as hydrogen bonds are broken

2. Pairing - complementary RNA nucleotides drift in and pair with one side of the DNA molecule (uracil replaces thymine)

3. Joining - RNA polymerase joins the nucleotides together into a single strand. DNA strands rejoin

B. DNA has two strands, coding and anticoding, only one side, the anticoding strand is used to make mRNA

C. DNA promoter region - defines start of a gene, direction of transcription and the strand to be copied

V.  Translation - takes place in cytoplasm on ribosomes

A. Transfer RNA (tRNA)

1. tRNA - transfer amino acids to ribosomes

2. tRNA is a single-stranded ribonucleic acid that doubles back on itself to create regions where complementary bases are hydrogen-bonded to one another

3. At one end it binds to amino acid; at other end it has an anticodon that binds to mRNA codon

4. Anticodon = three consecutive nucleotides on a tRNA molecule complementary to codon on mRNA

5. Charged tRNA = a tRNA carrying an amino acid

B. Ribosomes - sites of protein synthesis

1. Composed of rRNA and proteins

2. Two subunits, large and small

3. Occur free in cytoplasm or attached to endoplasmic reticulum

C. Mechanics of translation: 

    1. Initiation

    a. Small ribosomal subunit attaches to mRNA in vicinity of the start codon (AUG)

    b. First tRNA pairs with this codon; then large ribosomal subunit joins to small subunit

    c. Ribosomes contain two sites: the P (for peptide) site and the A (for amino acid) site

    d. First tRNA binds to P site, A site is for next tRNA.

    2. Elongation

a. second charged tRNA pairs its anticodon with the next codon on the mRNA

b. enzyme forms a peptide bond between the two charged amino acids

c. First tRNA, now uncharged, is ejected and recycled

d. Ribosome moves down the mRNA one more codon

e. another charged amino acid pairs its anticodon with the mRNA codon, and another peptide bond forms

f. Process continues until a stop codon occurs on the mRNA

      3. Termination

a. stop codon causes the process to stop

b. an enzyme releases completed polypeptide chain

c.  mRNA and the two ribosomal subunits separate

VI. Changes in genes

A. Gene = a sequence of DNA nucleotides that codes for a specific polypeptide, or regulates the transcription of such a sequence

B. Mutation = alteration in nucleotide sequence of DNA

1. Frameshift mutations occur when one or more nucleotides are inserted or deleted from DNA

2. Point mutation changes a single nucleotide and therefore changes a single specific codon

C. Cause and repair of mutations

1. Mutations due to DNA replication errors are rare

2. DNA polymerase constantly monitors, proofreads new strand against old, and repairs any irregularities, reducing mistakes to one out of every one billion nucleotide pairs replicated

3. Mutagens = environmental substances that cause mutations

a. Common mutagens are radiation, organic chemicals such as some pesticides and smoke

b. If mutagens cause mutation in gametes, then offspring may be affected

c. If the mutation occurs in the body cells, then cancer may result

d. UV radiation causes thymine dimers; adjacent thymines bond together

e. There are natural built in DNA repair mechanisms which cut out and replace incorrect nucleotide sequences. However only so much damage can be repaired

VII. Eukaryotic vs. prokaryotic genes

A. Eukaryotic genes differ from prokaryotes in that there are non coding regions within the genes called introns

B. The actual coding regions are called exons

C. This means that the non coding regions of the primary transcript must be cut out and the coding portions spliced together to make the mRNA

VIII. Try this on-line transcription/translation exercise

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