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Understand the process of transcription and translation in DNA and RNA, including the steps involved and the role of enzymes. Learn about gene expression and the synthesis of proteins.
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TranscriptionANDTranslation By Michael and Nick
Background Information • The GENOTYPE of an organism is its genetic makeup; described as the sequence of nucleotide bases in the organism’s DNA. • The PHENOTYPE of an organism is its physical traits; it is the result of many actions performed by different proteins. • Examples of proteins include enzymes of structural proteins.
Background Information • DNA codes for the building, or synthesis, of proteins. • Genes send the instructions for building proteins in the form of RNA. • RNA then codes for the synthesis of proteins. • “Central Dogma” is the term that describes this process. Term coined by Francis Crick.
Background Information • DNA is located in the nucleus of the cell. • TRANSCRIPTION occurs and DNA information is transferred to RNA. • TRANSLATION occurs and protein synthesis takes place in the cytoplasm of the cell.
Background Information • DEFINITIONS: • TRANSCRIPTION is defined as the transfer of genetic information from DNA into an RNA molecule. (page 178) • TRANSLATION is defined as the transfer of the information from RNA into a protein. (page 178)
Overview • DNA and RNA are both polymers. • The monomers of DNA and RNA are linked together in certain sequences that are used to convey information. • DNA has the nitrogenous bases labeled A, T, C, and G (on their nucleotides). • RNA has the nitrogenous bases labeled A, U, C, and G (on their nucleotides).
Overview • The double helix of DNA consists of linear sequences of nucleotide bases. • The genes on a strand of DNA are made up of the specific sequences of these bases.
Overview • The genetic information that is used to build the amino acid sequence of a polypeptide chain is written in DNA and RNA; it is written in the form of codons.
Overview • A codon is a three-nucleotide sequence located in DNA that is transcribed into a three-base codon in RNA; the codons are complementary. • The RNA codon is then translated into the amino acid sequence. Amino acid sequences eventually form a polypeptide.
Overview • Summary: one codon consists of three nucleotides. One DNA codon One RNA codon One amino acid
The first step of transcription involves two DNA strands separating at the location where the process will take place. • One strand of this DNA serves as the template for building the new molecule of RNA.
The new RNA molecule consists of nucleotides that take their places along the DNA strand (using the DNA strand as a template). • The nucleotides of RNA form hydrogen bonds with the nucleotide bases that are on the strand of DNA. • RNA nucleotides are then linked by RNA polymerase, the transcription enzyme.
RNA polymerase is defined as an enzyme that links together the growing chain of RNA nucleotides during transcription, using a DNA strand as a template. • The base-pairing rules that apply to DNA also apply to RNA, except that RNA has the base U rather than T.
DNA nucleotides form specific sequences that tell the RNA polymerase where it should start and stop the process of transcribing information. • There are three main steps that describe this process:
Step One: Initiation of Transcription • A promoter, which is located in the DNA, is a nucleotide sequence that signals RNA polymerase to begin transcribing (located at the start of the gene). • Initiation of Transcription: the attachment of RNA polymerase to the promoter and the start of RNA synthesis. (page 181) • The promoter also determines which of the two strands of DNA is to be transcribed on a gene.
Step Two: RNA Elongation • Elongation is the stage in which RNA grows longer. • As the RNA continues to be synthesized, it begins to peel off of the DNA strand. • The two DNA strands then come back together, forming the double helix that the DNA originally started with.
Step Three: Termination of Transcription • The terminator, or the special sequence of bases located in the DNA template, is reached by RNA polymerase. • The terminator signals the end of the gene. • RNA polymerase then “unhooks” itself from the molecule of RNA as well as the gene. • Transcription therefore produces the RNA that codes for the sequence of amino acids.
Processing Eukaryotic RNA • Eukaryotic cells perform transcription in the nucleus, but they also process the RNA transcripts in the nucleus before they go to the cytoplasm (where translation by the ribosomes takes place). • One type of RNA processing adds a cap and tail (nucleotides) to the ends of the RNA. This gives protection from cellular enzymes and also helps the ribosomes to recognize the RNA as being the messenger RNA (mRNA).
Processing Eukaryotic RNA • Genes include both introns (internal noncoding regions) and exons (coding regions; the parts of a gene that are expressed). Both are transcribed from the DNA to the RNA. • Before RNA leaves the nucleus, the introns are removed and the exons join together to form one strand: a “continuous coding sequence,” which makes up the mRNA molecule. (page 182) • This process is known as RNA splicing. The mRNA is now ready for translation.
The synthesis of a polypeptide using the genetic information encoded in an mRNA molecule.
The tools and processes that are used to translate mRNA need both enzymes and sources of chemical energy, including ATP. • Translation also requires transfer RNA (tRNA) and ribosomes.
Transfer RNA • The translation of genetic messages from mRNA into the amino acid “language” used by proteins relies on an interpreter: transfer RNA. • tRNA converts the codons of nucleic acids into the amino acid “languange” that is used to form proteins.
Transfer RNA • Cells that make proteins contain many amino acids in their cytoplasm. • The job of the tRNA molecules is to match the amino acids with the correct codons, in order to make the new polypeptide.
Transfer RNA • tRNA molecules must perform two functions to carry out this task: • Pick up the appropriate amino acids. • Recognize the appropriate codons in the mRNA. (page 183)
Transfer RNA • tRNA molecules are made of single strands of RNA that twist and fold in many different places, forming double-stranded regions. • The ends of the folded molecules are known as anticodons because they are a “special triplet of bases.” (page 183) • Anticodons are defined as specific sequences of three nucleotides that are complementary to a codon triplet on mRNA. (page G-2)
Ribosomes • Ribosomes, along with making the polypeptides, help the mRNA and the tRNA to function properly. • Ribosomes are made of two different subunits: protein and ribosomal RNA (rRNA) • Fully assembled ribosomes have binding spots for mRNA on their small subunit and binding spots for tRNA on their large subunit. (page 183)
Ribosomes • tRNA has two binding sites: • The P site, which holds the tRNA that carries the polypeptide chain as it grows. • The A site, which holds the tRNA that carries the next amino acid that is going to be connected to the chain. • Anticodons of tRNA then pair with the codons of mRNA, and they are held together by the two different subunits of the ribosome. (page 183)
Ribosomes • Finally, the ribosome is able to join the amino acid from the A site of the tRNA to the polypeptide chain, which is now growing. (page 183)
The Process of Translation Translation is also divided into three stages. The three stages are initiation, elongation, and termination.
Stage One: Initiation • Initiation brings the mRNA, the first amino acid (including its tRNA that is attached), and the two subunits of a ribosome together. (page 184) • This process determines the specific location where translation will begin, so that the mRNA codons get translated in the proper sequence of amino acids. (page 184)
Stage One: Initiation • This process occurs in two steps: • mRNA connects to the small ribosomal subunit and tRNA connects to the start codon. • The large ribosomal subunit joins with the small ribosomal subunit, and a working ribosome is established. The initiator tRNA also fits into the P site that is located on this ribosome. (page 184)
Stage Two: Elongation • Elongation is a three-step process: • Codon recognition takes place: the process in which tRNA’s anticodon joins with mRNA’s codon in the A site, bringing an amino acid with it. • Peptide bond formation takes place: the polypeptide connects to the amino acid in the A site of the tRNA molecule and the ribosome acts as a catalyst for the formation of the bond. • Translocation takes place: the ribosome now moves the tRNA (that remains) to the P site, and it brings the growing polypeptide with it as well; the tRNA and mRNA move together; the next mRNA codon is brought into the A site and the process begins anew. (page 184)
Stage Three: Termination • The stop codons in mRNA eventually reach the A site of ribosomes, telling the process of translation to stop. • The finished polypeptide is released and the ribosome then splits back into its subunits. (page 184)
The flow of genetic information in a cell: DNA RNA PROTEIN
Transcription: DNA to RNA • Takes place in the nucleus. • Processes RNA before it is able to enter into the cytoplasm.
Translation: RNA to Protein • Takes place in the cytoplasm. • While polypeptides are being made, they fold and coil. This is responsible for the tertiary structure of proteins. • Quaternary structure occurs when many polypeptides join together.
Overall Information in a Gene, or the certain sequence of nucleotides in DNA Gene causes the transcription of a complementary sequence of nucleotides in mRNA mRNA determines the sequence of the amino acids in the polypeptide Polypeptides form specific proteins The capabilities and the appearance of cells and organisms is established
Transcription and translation are the ways in which the genotype produces the phenotype. (page 185) • In other words, these processes allow genes to control both the structures and the activities of our cells. (page 185)
