Transcription Translation Protein Synthesis Biology 12 l l
Transcription & Translation Protein Synthesis Biology 12
l l l Genes direct the production of proteins that determine the phenotypical characteristics of organisms. Genes also direct the production of other physiologically essential proteins such as antibodies and hormones. Proteins drive cellular processes such as metabolism; determining physical characteristics and producing genetic disorders by their absence or presence in an altered form. Metabolism is a term that is used to describe all chemical reactions involved in maintaining the living state of the cells and the organism. Metabolism can be conveniently divided into two categories: Catabolism - the breakdown of molecules to obtain energy Anabolism – the synthesis of all compounds needed by cells )
The Central Dogma l l An organism’s genome is housed within the nucleus. Proteins are synthesized outside the nucleus, in the cytoplasm, on ribosomes. Since information for protein synthesis is specified by DNA (called the one gene-one polypeptide hypothesis), and DNA is not able to exist outside the nucleus, a problem exists as to how the blueprint of life is brought to the ribosomes.
The Connection Between Genes and Proteins Nucleic acids carry information in their nucleotide sequence. Proteins carry information in their amino acid sequence. To get from DNA (in nucleic acid language) to protein (in amino acid language) requires two steps. 1. Transcription- a DNA strand provides a template for the synthesis of a complementary RNA strand. This molecule is called m. RNA (messenger RNA). DNA is too valuable to be allowed to exit the nucleus. This could lead to the death of the cell and possible the Death of the organism.
- use of m. RNA provides protection for the Genetic information contained in DNA. - more protein can be made simultaneously because many m. RNA copies of a gene can be made than if one strand of DNA left the nucleus. - each m. RNA can be translated many times.
m. RNA delivers the encoded genetic material to the ribosomes. The ribosomes translate the message into polypeptide chains, which are processed into proteins. This entire sequence is described as the Central Dogma of Molecular Genetics, first stated by Francis Crick in 1958.
Central Dogma l In nucleus l l Produced in nucleus Travels to cytoplasm l Produced in cytoplasm
Transcription vs Translation Transcription involves the copying of the information in DNA into m. RNA. (copy from one medium to another- think of a medical or legal stenographer) Translation involves ribosomes using the Messenger RNA as a blueprint to synthesize a protein composed of amino acids. (converting into a different language, think English to French)
Definition: Transcription Nucleus Location DNA Template (What is read) To change DNA into a form that can make a protein Purpose Messenger RNA (m. RNA) Outcome (End result)
Definition: Translation Location Cytoplasm (by ribosome) Template (What is read) m. RNA Purpose Amino acids assembled in particular order to make a protein Outcome (End result) Protein (polypeptide)
Central Dogma
RNA RNA: How is it difference from DNA? - contains a ribose sugar - contains the base Uracil (not thymine) - single stranded - found in both nuclues and cytoplasm Purine Bases (double ring) Adenine & Guanine Pyrimidine Bases (single ring) Cytosine & Uracil Base Pairs: (purine always pairs with pyrimidine) Adenine + Uracil Guanine + Cytosine Image: www. biologycorner. com/bio 1/DNA. html
Types of RNA Genetic information copied from DNA is transferred to 3 types of RNA: Messenger RNA: m. RNA Copy of information in DNA that is brought to the ribosome and translated into protein by t. RNA & r. RNA Varies in length , the longer the gene the longer the m. RNA> Transfer RNA: t. RNA Brings the amino acid to the ribosome that m. RNA coded for. Ribosomal RNA: r. RNA Most of the RNA in cells is associated with structures known as ribosomes, the protein factories of the cells. Provides the construction site for the assembly of polypeptides. It is the site of translation where genetic information brought by m. RNA is translated into actual proteins. Transcription & Translation
Transcription occurs in 3 steps: l l Initiation, Elongation and Termination Initiation: RNA polymerase binds to the DNA at a specific site known as a promotor. DNA: RNA: A T G C A A U A C G U U The RNA transcript is known as elongation. After the RNA polymerase passes the end of the gene, it stops transcribing which is termination.
Transcription : ‘to copy’ Initiation: l RNA polymerase binds to DNA at ‘promoter’ l untwists the double helix 10 to 20 bases at a time Elongation: l RNA polymerase builds m. RNA l l From DNA 3’ end Uses complimentary base pairing l Remember: thymine (T) is replaced by uracil (U)
Termination: l l l RNA polymerase reaches end of gene. Stops transcribing Double helix reforms as m. RNA molecule peels away. End Result: m. RNA breaks away from DNA l m. RNA exits nucleus l If there is a high demand for a protein, the cell can have several RNA polymerases transcribing the gene at the same time to produce several m. RNA’s. l
Translation: ‘new language’ Initiation: l Ribosome binds at a specific sequence on the m. RNA. l The ribosome moves along the m. RNA three nucleotides at a time. This is called a codon. Each set of three (a codon) codes for an amino acid. Why? There are only 4 bases but 20 amino acids. 41 = 4 (1 base=1 acid) 42 = 16 43 = 64
The codon AUG not only codes for the amino acid Methionine, but it also indicates the start of a translation. Some amino acids are coded for by two or more codons but a given codon ALWAYS only codes for one amino acid. GAA and GAG both code for glutamic acid, but never mean any other amino acid.
Elongation: Ribosome moves along m. RNA l From m. RNA 5’ end l 3 nucleotides of m. RNA = codon = amino acid l The “interpreter” t. RNA delivers the proper complimentary base to the ribosome. Anticodons are blocks of 3 t. DNA bases that actually attach to the correct protein. l The anticodon( t. RNA) binds by complimentary base pairing to the nucleotides of the codon. l Example: if the codon on a m. RNA is UUU, a t. RNA with an AAA anticodon will bind to it. The ribosome links adjacent amino acids with a peptide bond, causing the amino acid to let go of it t. RNA. The finished protein has a sequence of amino acids that have been determined by the m. RNA base sequence which has been translated by the t. RNA. l
l l The ribosome then adds each amino acid and the polypeptide chain is elongated. Elongation occurs until a stop signal occurs. Termination: l Ribosome reaches stop codon l Stops translating End Result: l Ribosome falls off m. RNA l Protein (polypeptide chain) is released
Start and Stop Codons Start Codon: l Begins translation l AUG (universal start codon) l l ALSO Codes for methionine (Met) Sometimes GUG or UUG Stop Codon: l Ends translation l UGA, UAG
The Whole Picture Next amino acid to be added to polypeptide Growing polypeptide t. RNA m. RNA
Example l DNA template: 3’ TAC ACA CGG AAT GGG TAA ACT 5’ l Complimentary DNA l l m. RNA codon l l Read from DNA template (start reading at 3’) t. RNA anticodon l l Read from DNA template (start reading at 3’) Read from m. RNA Amino Acids (protein) l Read from m. RNA
Task A: #2 – Central Dogma l DNA makes RNA (m. RNA) l l through transcription RNA makes proteins l through translation
#4 – RNA types l m. RNA l l Messenger RNA End product of transcription Takes message from DNA into cytoplasm Used by ribosome to make protein l t. RNA l l l Transfer RNA Delivers amino acid to ribosome r. RNA l l Ribosomal RNA Helps form and maintain ribosomes
#5 – DNA vs. RNA DNA RNA l Sugar – deoxyribose l Sugar – ribose l Double stranded l Single Stranded l Base pair – thymine l Base pair – uracil l Stays in nucleus l Can leave nucleus l Can replicate itself l Cannot replicate itself l Longer strands l Shorter strands
#6 – Transcription/Translation Transcription l Purpose: l To make m. RNA from DNA l Location: l Nucleus Translation l Purpose: l To make a specific protein from m. RNA l Location: l Cytoplasm (ribosome)
#9 – Stop vs. Start Codon l m. RNA code l Tells ribosome to begin translation l Example: l AUG Also codes for methionine And: UUG, GUG l l Stop Codon l m. RNA code l Stops translation of that specific amino acid chain l Examples: l UAA, UAG, UGA
#10 – Transcribe to m. RNA l DNA: GGA TCA GGT CCA GGC AAT TTA GCA TGC CCC AA l *m. RNA*: CCU AGU CCA GGU CCG UUA AAU CGU ACG GGG UU
#11 – Translate to Amino Acids l m. RNA sequence divided into codons: GGC AUG GGA CAU UUU GCC CGU UGU GGG GCG UGA l *Protein translation*: Gly Met(start) Gly His Tyr Phe Ala Arg Cys Gly Ala (stop)
Task B: #2 – Transcribe to m. RNA l DNA: TAC GGT AGG TAT A l *m. RNA*: AUG CCA UCC AUA U
Task C: #3 – Anticodons
#4 – Change in 3 rd Base May Not Result in Error l Why not? l l Amino acids have more than one codon Example: proline l l l Codons CCU, CCC, CCA, and CCG CC - always codes for proline Third base/nucleotide does not matter
#6 – Translate to Amino Acids l m. RNA: GGC CCA UAG AUG CCA CCG GGA AAA GAC UGA GCC CCG l *Protein translation*: Met (start) Pro Gly Lys Asp (stop)
- Slides: 40
