Read the pages indicated by D-12 concerning DNA replication. Study behavioral objectives 29-31. 

OVERVIEW: 

Recall that DNA is double stranded, twisted in the form of a helix. The two DNA strands of each double helix unwinds. One side of the double strand will uncouple from the opposite strand (unzips). While these two strands are apart individual nucleotides will match up to complementary partners.  With the aid of an enzyme DNA polymerase, which moves along each strand of the nucleotides, will hook together by condensation. You learned the condensation reaction in an earlier miniunit. 

END RESULT: 

After replication two double helices are synthesize, each consisting of one parental DNA strand plus one newly synthesized complementary strand. the new strand  is an exact copy of the other parental strand. The two daughter DNA molecules are therefore duplicates of the parental DNA molecule. 

After reading  the pages in the text write out a step by step description of events which led to the synthesis of a DNA molecule.  Include the following terms: 
helicies, polymerases,  semiconservative replication and 3' to 5' direction. 
 

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2. Semiconservative replication refers to the fact that: 

    a) only certain bases  pair to specific bases 
    b) one DNA strand remains while the other is newly synthesized 
    c) only the newly synthesized DNA contains genes 
    d) both DNA strands contain newly synthesized nucleotides 
 

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3. DNA ligase: 

   a. connects backbones of newly synthesized polynucleotide. 
   b. initiates the unwinding of a double helix. 
   c. directs free nucleotides to exposed bases. 

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4. If the bases in the parent DNA strand are  AATCATGGC what would be the correct base sequence located in the complimentary strand. 

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When does replication occur in a working cell?  This is the process which occurs during the "S" stage of cell division. You may want to go back and review that part of mitosis and meiosis. 
 
 

TRANSCRIPTION:  
 

Read pages indicated by D-13 and study behavioral objectives 28, 32, 35, and 36. 
 
 

OVERVIEW:  

During transcription DNA will synthesize RNA. Three types of RNA will be synthesized: mRNA (messenger), tRNA (tranasfer) and rRNA (ribosomal). During the transcription of mRNA the information contained in a DNA segment  (gene) is copied into a mRNA strand. This process is similar to DNA replication except only a small segment of DNA is  copied, and the nucleotides which are bonded are the ones which make up RNA. Recall from the previous lesson that  RNA nucleotides consist of the sugar ribose in place of deoxyribose, and the base A,U,C,G in place of A,T,C,G. 
 
 

END RESULTS:  Only one strand of the double stranded DNA  forms a complimentary strand of mRNA.  This RNA  represents only a small portion of the entire DNA (gene).  It contains the sugar ribose in place of deoxyribose and the same bases as the DNA parent strand except "U" in the place  of "T". 
 

5. After reading  the pages in the text write out a step by step description of events which lead to the synthesis of a mRNA molecule.  Include the following processes: section of DNA transcribed, template strand, RNA polymerse, promoter, initiation, elongation of RNA molecule, and termination. 
 

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6. If the bases on the template DNA strand for a gene are TTTAATCATGGC,  what would be the correct base sequence of the mRNA synthesized by transcription? 

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The tRNA and the rRNA are transcribed in a similar manner. The genes which code for the synthesis of rRNA are located in the nucleolus and the genes which  code for the synthesis of tRNA are located in chromatin DNA. 

All three of these RNA's are transcribed in the nucleus and must move into the cytoplasm where they will coordinate the efforts which will lead to the synthesis of a specific protein directed by the mRNA.  These are very large molecules and will move into the cytoplasm though the nuclear pores. 
 
 
 
 

TRANSLATION: 
 

Study behavioral objectives 33, 34 and 37 and read the pages indicated by D-14 

Translation is the synthesis of a protein which is directed by a code inscribed on the mRNA molecule.  Translation occurs on the ribosome in the cytoplasm and requires the use of the three RNA's: mRNA, tRNA and rRNA (see B.O. 32). 
 
OVERVIEW: 

Only the mRNA carries the code from the gene to synthesize a specific protein. During the process ribosome's (made up of protein and rRNA) act as a stage for the following specific events: tRNA  picks up specific amino acids located in the cytoplasm and  transfers them to the ribosome, mRNA contains the code which is decoded by the tRNA and places specific amino acids in close proximity in order for them to bond together by condensation to form a protein. 

Fill out the following table and place in you notes. 
 
 

END RESULTS:  

A protein with a specific primary structure will be formed. The specific primary structure is directed by the gene (DNA segment) which was transcribed to a mRNA. 

After reading  the pages in the text, write out a step by step description of events which occurs during translation.  Include the following processes: location of  mRNA and tRNA on the ribosome, initiator tRNA, codon, anticodon, protein elongation, stop codons. 

SUMMARY OF EVENTS: 

mRNA with code for amino acid sequences of protein attaches to the ribosome. 

Ribosome's consist of rRNA and protein. They bind with the mRNA and part of the tRNA with corresponding amino acids. 

tRNA consist of three exposed nitrogen bases (anticodons) at one end and a specific amino acid at the opposite end. 

As the mRNA slides along the ribosome specific tRNA, they (anticodon) match up to the mRNA (codons) exposed on the ribosome. 

Example: 
mRNA with the base sequences "AUGAAACCC" represents three codons and would match up with three tRNA with the following anticodons - UAC, UUU, GGG. 

The amino acids carried to the ribosome by  tRNA will be linked together by condensation reactions learned in an earlier unit. 

The first initiator tRNA "UAC" will move from the cytoplasm to the AUG codon of the mRNA which has positioned itself on the ribosome. The second  tRNA "UUU" with its corresponding amino acid  will move from the cytoplasm to the "AAA"  codon of the ribosome. The two amino acids will link together by condensation reaction.  The first initiator tRNA will break away from the DNA leaving its amino acid attached to the second amino acid still attached to the tRNA with the "UUU" anticodon. 

As the mRNA moves along  the ribosome a third  tRNA "GGG" with its amino acid  attaches to the DNA by the anticodon/codon attraction. 

This amino acid will linked to the end of the dipeptide present and the second tRNA will move from the parent DNA 

These steps are repeated over and over until  a codon with a STOP message appears. 
 
The STOP messages ends the synthesis of the polypeptide. 
 

Because the codons are in triplets there are 64 different combinations of anticodon and codon.  There are approximately 20 amino acids. Therefore, some of the amino acids may be associated with tRNAs with different anticodon.. 

Study the Genetic Code (codons of mRNA) table below. Learn to read this table. Note that the triplets refer to the codons on the mRNA and not the anticodons of the tRNA. 
 
                   codon table  
 

7. Which RNA   contains codons? 
 
 

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8. Which  RNA  contains anticodons? 
 

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9. How many bases are in a codon or anticodon? 
 

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10. What is the codon  for the initiator tRNA? 
 

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11. If a gene had the following base sequence TACCCTCAAGATGCGTTC, 
what would be the sequence of bases on the mRNA? 

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12. Which RNA's would be used to bring the specific amino acids to the ribosome?   Remember this table represents the codon and not the anticodon. 

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13. Using the Genetic Code table determine the primary structure of the protein formed (sequence of amino acids)? 
 
                              codon table 

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14. What would be the base sequence for the gene which forms the following 
polypeptide (include start and stop)? 
 methionine-tyrosine-lysine-phenyalanine-phenylalanine-cysteine-glycine-lysine 
 
                          codon table  

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15. Identify the amino acids which is associated with the anticodon  UUU. 
 
 
 
 
 
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16. Identify the amino acid which is associated with the codon AAC. 
 
 
 
 
 
 

           codon 
 
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Study behavioral objectives  25.  A specific protein (sequence of amino acids)  is formed during translation. The orderly sequence of amino acids were directed by a segment of DNA (gene). The DNA formed a complementary mRNA during transcription.  The mRNA's  arrangement of nitrogen bases (codons)  attracted specific tRNA due to their anticodons during translation. The protein formed  takes on a particular three dimensional shape depending on  its primary structure which dictates proteins  secondary structure, tertiary structure and quaternary structure if present.  At this point the protein is called an enzyme. This enzyme will in turn catalyze a specific reaction causing a particular phenotype, such as red flower color. 

Relate the paragraph above to the figure below. 

 
 
 
 
 
Write the function for the following terms as to their roll in translation. 

         RNA polymerase - 

         promoter region of gene - 

         start codon - 

         stop codon - 
 
 
MUTATIONS  
 
Study behavioral objective 24 and read the section indicated by D-16. 204-207 in the text. 

In your notes write a paragraph contrasting between  point mutations, insertion mutations and deletion mutations. 

Write a paragraph explaining the effect of mutations on protein structure and function. 

In summary, how do genes work in producing the various phenotypes which make up all living organisms, bacteria, plants and animal? Study the following statements and write in your notes. 

 Let's work backwards starting with the specific phenotype. For example, what causes some individuals to have facial freckles?  This phenotype is caused by specific chemical reactions that occur in some of the epidermal cell of that individual. During this process the pigment melin is synthesized. 

       Protein Synthesis 

What controls these reactions?  In an earlier miniunit you learned that chemical reactions in living cells are controlled by enzymes. Also you learned that enzymes are made up of protein and have specific three dimension structure dictated by their primary structure (sequence of amino acids in the polypeptide). 

How are these specific polypeptide (protein) synthesized? Proteins are synthesized at the ribosome's in the cytoplasm during the process of translation. 
Translation is where a specific mRNA (contains a specific arrangements of nucleotides) are decoded by a tRNA. The tRNA molecule has specific amino acids attached at one end in relation the the anticodon (triplet bases) at a different location. These anticodons match up with triplet codons located on the mRNA. Therefore the codons on the mRNA are directing the synthesis of a specific protein. Where did  mRNA receive this message (arrangement of nucleotides forming the triplet codon)?  The mRNA was copied from a small segment  of DNA (gene) during the process of transcription. 

 
 
 This is the end of miniunit Delta. Go back to the home page and examine the www pages concerning miniunit Delta. They may be found in the resource section. You may now take the practice test for miniunit Delta. Click here to go back to the Biology 108N home page and review or take the practice test.  Good Luck! 
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