Transcription - G10 PDF

Transcription Objectives At the end of the topic lesson, the learners should have: -diagrammed the steps in transcription and translation. What are proteins? They are composed of amino acids linked together by peptide bonds. Roles and functions of proteins: channels in membranes –...

Transcription Objectives At the end of the topic lesson, the learners should have: -diagrammed the steps in transcription and translation. What are proteins? They are composed of amino acids linked together by peptide bonds. Roles and functions of proteins: channels in membranes – control the movement of molecules in and out of the cell structural molecules – for example, making up hair or muscle in animals hormones – to regulate the activity of cells antibodies – in the immune system enzymes – to act as catalysts in biological systems. Protein synthesis Protein synthesis refers to the biological process whereby amino acids are assembled by peptide bonding into specific polypeptide sequences in accord with genetic blueprints encoded by deoxyribonucleic acid (DNA). Steps in protein synthesis: Transcription Translation DNA and RNA DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) work together to produce proteins from genetic codes. Genetic codes are found in DNA or RNA which is made up of nucleotide bases usually in three’s (triplet) that code for the amino acids making up the proteins. Seatwork: 15 minutes Contrast DNA and RNA according to their type of sugar, structure, stability under certain conditions, functions, and nitrogenous bases. ½ crosswise paper Worth 20 points Open notes Can access the internet A short summary of the difference between DNA and RNA. RNA RNA (Ribonucleic Acid), unlike the double stranded DNA, is a nucleic acid polymer with a single strand. It is composed of the four nucleotides adenine, uracil (replaced thymine in DNA), guanine and cytosine which are represented by their first letter A, U, G, C. (The only difference with DNA is the Uracil). RNA is the first intermediate in converting the information from the DNA into proteins which is important for proper cellular function. Thymine vs Uracil The main distinction between thymine and uracil lies in their chemical structure. Thymine has a methyl group (CH3) attached to its ring structure, whereas uracil does not have this methyl group. This structural difference is responsible for the various roles of thymine in DNA and uracil in RNA. Uracil is the demethylated form of thymine. Stability Thymine, with its methyl group, improves the stability and structural reliability of DNA. The methyl group helps protect the DNA molecule from chemical damage and enzymatic destruction. It also plays a character in holding errors in DNA replication and maintaining the overall structure of the DNA double helix. Stability under alkaline conditions Due to its deoxyribose sugar, which contains one less oxygen- containing hydroxyl group, DNA is a more stable molecule than RNA, which is useful for a molecule which has the task of keeping genetic information safe. RNA, containing a ribose sugar, is more reactive than DNA and is not stable in alkaline conditions. Functions RNA falls into three major categories: Messenger RNA (mRNA) Ribosomal RNA (rRNA) Transfer RNA (tRNA) mRNA mRNA copies the genetic code from the DNA into a form that can be read and used to make proteins. mRNA transmits genetic information from the nucleus to the cell’s cytoplasm. rRNA rRNA is situated in the cytoplasm of a cell, where we can find the ribosomes. rRNA leads the translation of mRNA into proteins. tRNA tRNA transfers amino acids to the ribosome that matches to each three-nucleotide codon of rRNA. The amino acids then can be combined together and processed to make polypeptides and proteins. 1. Messenger RNA (mRNA) - transcribes the DNA nucleotide bases to RNA nucleotide bases 2. Ribosomal RNA (rRNA) - binds the mRNA and tRNA to ensure that codons are translated correctly 3. Transfer RNA (tRNA) - translates the mRNA codons into the correct amino acids Transcription Transcription in protein synthesis is the process where RNA is made from the DNA by copying the base sequence of the double stranded DNA into a piece of a single stranded nucleic acid. This transcription process is catalyzed by the enzyme RNA Polymerase. Location Transcription of DNA to form RNA takes place in the cell’s nucleus. This process uses DNA as a model to make an RNA (mRNA) molecule. During transcription, a strand of mRNA is made that corresponds to a strand of DNA. Just like DNA replication, transcription also occurs in three major steps: initiation, elongation and termination. Why does it happen in the nucleus? Initiation Initiation is the start of transcription. It transpires when the enzyme RNA polymerase binds to a specific region of a gene which is called the promoter with the help of proteins called ‘transcription factors’. This signals the DNA double strand to unwind and open so the RNA polymerase enzyme can ‘‘read’’ the bases found in one of the DNA strands.  With the open strands, one is considered as the template strand (anti-sense strand) and this will be used to generate the mRNA. The other is called the non-template strand (sense strand). After reading the bases, the RNA polymerase enzyme is now ready to make a strand of mRNA with a complementary sequence of bases. Elongation Elongation is the adding of nucleotides to the mRNA strand. RNA polymerase reads the opened DNA strand and forms the mRNA molecule with the use of complementary base pairs. There is a short time during this process when the newly formed RNA is bound to the opened DNA. During this process of elongation, an adenine (A) in the DNA binds to an uracil (U) in the RNA.  RNA polymerase does not need a primer during this process. It simply initiates the mRNA synthesis from the starting point and then moves downstream reading the anti-sense strand from 3’ to 5’ and generating the mRNA from the 5’ to 3’ end as it goes. Unlike helicase enzyme in DNA replication, RNA polymerase zips DNA back up as it goes keeping only 10-20 bases exposed one at a time. Termination Termination is the last step of the transcription process. This happens when RNA polymerase enzyme reaches a stop or termination sequence in the gene. When the stop sequence or stop codon is reached, the enzyme detaches from the gene. The mRNA strand is now produced and it detaches from DNA.  It carries with it the information encoded in the gene. By the end of transcription, the DNA segment is transcribed to form the mRNA molecule. The template strand shown with the sequence T-A-C-T-A- G-A-G-C-A-T-T transcribes to form the mRNA A-U-G-A-U- C-U-C-G-U-A-A. Remember! Remember to take note of the transcription pattern: Thymine to Adenine, Adenine to Uracil, Cytosine to Guanine, Guanine to Cytosine. Uracil is being synthesized instead of Thymine as compared during DNA replication. Transcribe your DNA to RNA Identify the correct mRNA sequence from the given DNA template sequence. Answers 1. AUG ACU AGC UGG GGG UAU UAC UUU UAG 2. AUG GCG AGG CGG CAG CUG UUA UGG UGA 3. AUG GUG GGG GCA UAC CGA CCC UUA UAG 4. AUG AGA GGG UUU UUU AUG GUG GGG UAG 5. AUG GAG UGU GAU GCG UAC AAC CCC UAA What will happen to the transcribed mRNA?  For a protein-coding gene, the messenger RNA carries the information needed to build a polypeptide. The messenger RNA (mRNA) is the RNA form of the gene that leaves the nucleus through the nuclear pore and moves to the cytoplasm where proteins are made. Take note, transcription is the process wherein the DNA sequence of a gene is "rewritten" using RNA nucleotides. Now, it’s ready to be translated.