An Overview of Protein Synthesis

Genes

• A sequence of nucleotides in DNA that performs a specific function such as coding for a particular protein

Proteins

• Complex molecules composed of one ore more polypeptide chains made of amino acids and folded into specific three dimensional shapes that determine protein function

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Amino Acids

• The monomer unit of a polypeptide chain that is composed of a carboxylic acid, an amino group, and a side group that differentiates it from other amino acids

Transcription

• The process in which DNA is used as a template for the production of complementary messenger RNA molecules

Translation

• The process by which a ribosome assembles amino acids in a specific sequence to synthesize a specific polypeptide coded by messenger RNA.

Messenger RNA

• The end product of transcription of a gene, mRNA is translated by ribosomes into protein.

Transfer RNA

• A form of RNA that is responsible for delivering amino acids to the ribosomes during the process of translation.

Ribosomal RNA

• A form of RNA that binds with ribosomal protein to form ribosomes.

RNA Polymerase

• Enzyme that transcribes DNA into complementary mRNA.

Codon

• Sequence of three bases in DNA or complementary mRNA that serves as a code for a particular amino acid.

Start Codon

• Specific codon (AUG) that signals to the ribosome that translation commences at that point.

Stop Codon

• Specific codons that signal the end of translation to a ribosome.

1. Where is the information for protein synthesis found and where are the proteins synthesized?

• The information required for protein synthesis is found on the genome (DNA) inside the nucleus.

• Protein synthesis occurs in the cytoplasm of the cell and is carried out by ribosomes.

2. Why does DNA not leave the nucleus?

• DNA carries the genetic code for an organism which makes it incredibly valuable to the cell and its ability to carry out its functions.

• If DNA were to leave the protection of the nucleus it could be damaged or cut up by enzymes in the cytoplasm. This could render the DNA nucleus and normal cell functions would cease which would lead to the death of the cell and possibly of the organism itself.

3. What is the advantage of using RNA in transcription?

• Ribonucleic acid or RNA is used in place of DNA during transcription because DNA cannot leave the nucleus and there is not enough copies of each gene to provide the volume of proteins required by the cell in a timely manner.

• The complications of using DNA directly in protein synthesis are many. Instead, DNA can be easily transcribed into RNA repeatedly which can then be translated into proteins.

4. What is the difference between transcription and translation?

• Transcription takes place inside the nucleus and it is a process that involves copying the information from DNA into RNA.

• Translation involves the use ribosomes. The ribosome reads the code on the mRNA and translates that information into the synthesis of proteins.

5. Describe the characteristics and functions of the three major classes of RNA.

• mRNA is a single stranded molecule that varies in length depending on the gene that has been transcribed.

• tRNA is responsible for transferring the correct amino acid to the ribosome to build a polypeptide (protein). tRNA is fairly short, only 70-90 ribonucleotides in length.

• rRNA is a structural component of ribosomes along with proteins.

6. What are the three sequential processes in transcription?

• Transcription occurs in three phases known as initiation, elongation and termination.

7. What is the function of RNA polymerase during the initiation process of transcription?

• During initiation, RNA polymerase is responsible for starting or initiating transcription.

• It binds to the DNA template at a specific spot known as the promoter, which is near the beginning of the gene.

• RNA polymerase then uses the DNA template to build a strand of mRNA using ribonucletides. (elongation).

• It will stop when it receives a signal (termination).

8. During translation, why does the ribosome move along the mRNA and read it three nucleotides at a time?

• The genetic code is read in triplets or what is known as codons.

• Each set of three nucleotides codes for a specific amino acid.

9. When will the ribosome stop reading the mRNA?

• A ribosome will stop reading the mRNA when is reads the stop codon (UAA, UAG UGA).

• This is known as the termination stage of translation and the ribosome will divide into it’s two subunits and release the mRNA and the newly synthesized polypeptide.

10. What is a codon? How many different codons are there in mRNA?

• Codons are triplets of nucleotide bases that code for specific proteins.

• mRNA contains 64 different codons that code for the 20 different amino acids found in proteins.

11. Why is it okay that a redundancy exists in the genetic code, in other words, more than one codon can code for a specific amino acid?

• The redundancy in the genetic code attempts to minimize or reduce the errors that may lead to serious mutations.

12. What is the function of the stop and start codon?

• The start codon (AUG) is responsible for initiation of translation. It codes for the amino acid methionine which is always the first amino acid in all polypeptides.

• The stop codons (UGA, UAA, UAG) signal termination of translation. The ribosome will release the polypeptide after reading the stop codon.

13. If 61 codons code for the 20 different amino acids, what are the other three codons for?

• The other three are the stop codons that signal termination of translation.