T OPIC 3.5: T RANSCRIPTION AND T RANSLATION (P ROTEIN S YNTHESIS )

A SSESSMENT S TATEMENTS Compare the structure of RNA and DNA Outline DNA Transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase Describe the genetic code in terms of codons composed of triplets of bases Explain the process of translation, leading to polypeptide formation Discuss the relationship between one gene and one polypeptide

P ROTEIN SYNTHESIS I NTRODUCTION The control that DNA has over a cell is by a process called Protein Synthesis DNA controls the proteins produced in a cell Some of these proteins are Enzymes The Production or Lack of Production of a particular enzyme can have a dramatic effect on the overall biochemistry of the cell DNA indirectly controls the biochemistry of carbohydrates, lipids, and nucleic acids by the production of enzymes

P ROTEIN SYNTHESIS I NTRODUCTION Protein synthesis has two major sets of reactions Transcription (DNA to RNA) Translation (RNA to Polypeptides (protein)) Both Processes require RNA

C OMPARING DNA AND RNA DNARNA Contains a 5 carbon sugar 5 carbon sugar is deoxyribose5 carbon sugar is ribose Each nucleotide has one of four nitrogenous bases The nitrogenous bases are cytosine, guanine, adenine, and thymine The nitrogenous bases are cytosine, guanine, adenine, and uracil Double-stranded moleculeSingle-stranded molecule

T RANSCRIPTION PRODUCES RNA MOLECULES Genes-are the sections of DNA that codes for polypeptides Each gene is a specific sequence of nitrogenous bases found in a DNA molecule DNA is found inside nucleus but proteins are synthesized outside the nucleus in the cytoplasm Messenger RNA (mRNA) is an intermediary molecules which caries the message of DNA to the cytoplasm where he enzymes, ribosomes, and amino acids are found In the nucleoplasm contains free RNA nucleotides along with free DNA nucleotides

T RANSCRIPTION PRODUCES RNA MOLECULES Transcription Process: Begins when an are of DNA of one gene becomes unzipped (similar to the unzipping of DNA replication) Only one strand of the two DNA strands will be used as a template to create the mRNA molecule. RNA polymerase—enzyme used as the catalyst for this process RNA polymerase moves along the strand of DNA acting as the template, nucleotides float into place by complementary base pairing

T RANSCRIPTION PRODUCES RNA MOLECULES The complementary base pairs are the same as in double-stranded DNA, which an exception that adenine on DNA is now paired with uracil on the newly forming mRNA.

T RANSCRIPTION PRODUCES RNA MOLECULES Facts about Transcription: Only one of the two strands of DNA is ‘copied’, the other strand is not used mRNA is always single-strand and shorter than the DNA that it is copied form as it is a complementary copy of only one gene The presence of thymine in a molecule identifies it as DNA The presence of uracil in a molecule identifies it as RNA

T HE GENETIC CODE IS WRITTEN IN TRIPLETS The mRNA molecule produced by transcription represents a complementary copy of one gene of DNA This sequence of nucleotides making up the length of the mRNA is typically enough information to make one polypeptide The message written into the mRNA molecule is the message that determines the order of the amino acids

T HE GENETIC CODE IS WRITTEN IN TRIPLETS The Genetic Code is written in a language of three bases Three bases is enough to code for 1 of the 20 amino acids Triplet-any set of three bases that determine the identity of one amino acids When a triplet is found on the mRNA molecule, it is called a codon or codon triplet

T RANSLATION RESULTS IN THE PRODUCTION OF A POLYPEPTIDE There are three different types of RNA molecules. They are all single stranded and each is transcribed from a gene of DNA mRNA: each mRNA is a complementary copy of DNA gene and is enough genetic information to code for a single polypeptide rRNA: ribosomal RNA, each ribosome is compsed of rRNA and ribosomal protein tRNA: transfer RNA, each type of tRNA transfers 1 of the 20 amino acids to the ribosome for polypeptide formation

T YPICAL T RNA The three bases in the middle loop are called the anticodon bases and they determine which of the 20 amino acids is attached to the tRNA

T RANSLATION RESULTS IN THE PRODUCTION OF A POLYPEPTIDE Once an mRNA molecule has been transcribed, the mRNA detaches from the single-strand DNA template and floats free in the nucleoplasm. At some point, the mRNA will float through one of the many holes in the nuclear membrane (nuclear pores) and will then be in the cytoplasm

T RANSLATION P ROCESS The mRNA will locate a ribosome and align with it so that the first two codon triplets are within the boundaries of the ribosome The next step is the introduction of the tRNA. The tRNA must be complementary to the first codon triplet of the mRNA molecule Than, the first amino acid is brought into the translation process While the first tRNA ‘sits’ in the ribosome holding the first amino acid The second tRNA floats in and brings a second (specific) amino acid The second tRNA matches its three anticodon bases with the second codon triplet of the mRNA

T RANSLATION P ROCESS Two specific amino acids are now being held side by side An enzyme now catalyses a condensation reaction between the two amino acids and the resulting covalent bond between them is called a peptide bond

T RANSLATION P ROCESS The next step in the translation process involves the breaking of the bond between the first tRNA molecule and that amino acid that it transferred in The first tRNA floats away into the cytoplasm and invariably reloads with another amino acid of the same type

T RANSLATION P ROCESS The ribosome that has only one tRNA in it now moves one codon triplet down the mRNA molecule This creates room for a third tRNA to float in bringing with it a third specific amino acid This process continues until the ribosome gets to the last codon triplet—the final codon triplet will not code for an amino acid, but signals a stop codon

T RANSLATION P ROCESS The entire polypeptide breaks away from the final tRNA molecule, and becomes a free floating polypeptide in the cytoplasm of the cell

T HE O NE G ENE /O NE P OLYPEPTIDE HYPOTHESIS Beadle and Tatum set out to provide experimental proof of the connection between genes and enzymes. They hypothesized that if there really was a one- to-one relationship between genes and specific enzymes, it should be possible to create genetic mutants that are unable to carry out specific enzymatic reactions.

T HE O NE G ENE /O NE P OLYPEPTIDE HYPOTHESIS In the last few years, researchers have discovered that at least some genes are not quite that straight forward. For example, one gene may lead to a single mRNA molecule, but the mRNA molecule may be modified in many different ways Each modification may result in the production of a different polypeptide during the translation portion of protein synthesis