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FROM DNA TO PROTEIN Transcription – Translation We will use:

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1 FROM DNA TO PROTEIN Transcription – Translation We will use:

2 I. Overview Although DNA and the genes on it are responsible for inheritance, the day to day operations of the cell are done by proteins. The information from DNA to determine what type of proteins need to be made in each cell has to be moved from the genes to the ribosomes in the cytoplasm. The molecules that carry, edit and regulate this information to make proteins are various forms of RNA

3 Central Dogma of Molecular Biology The flow of information in the cell starts at DNA, which replicates to form more DNA. Information is then transcribed into RNA, and then it is translated into protein. The proteins do most of the work in the cell. Information flows in the other direction only in cases of some viruses and prions.

4 Points to review: Difference between DNA and RNA Cell organelles – nucleus, ribosomes RNA molecules are smaller polynucleotide chains that are formed from ribose, nitrogen- containing bases (A, C, G and U) and phosphate. They are usually single stranded Some types of RNA: Messenger RNA (mRNA) – carries information Transfer RNA (tRNA) – carries amino acids Ribosomal RNA (rRNA) – forms ribosomes


6 II. Transcription Transcription – the copying of a part of a DNA molecule to make an mRNA (messenger RNA) molecule. Active transcription occurs in specific parts of the nucleus The process of transcription: 1. Initiation – Transcription protein factors bind to the promoter region of the DNA molecule. This region is recognized by the presence of a TATA box

7 2. The transcription factors help RNA polymerase enzyme to find the proper region to be copied. 3. After binding, RNA polymerase begins to unwind the DNA double helix – the template chain of the DNA molecule will be copied. 4. Elongation – RNA polymerase assembles the RNA nucleotides that complement the template DNA nucleotides. This enzyme also reads from the 3 – 5 direction and builds the new RNA strand in the 5-3 direction. 5. As the RNA polymerase continues to copy the new strand, the copied section of the DNA molecule joins back because the RNA chain separates from the template.

8 6. Termination – The transcription continues until the enzyme encounters a terminator signal – a sequence of nucleotides that signals the end of transcription. 7. After the terminator nucleotides are transcribed the pre-mRNA molecule is released. 8. When the pre-mRNA is released the DNA reforms its original double helix structure. Game:


10 III. mRNA Processing Process that protects and allows the mRNA to leave the nucleus. Before the mRNA leaves the nucleus, its ends are modified with a 5cap and a polyA tail to protect it from damage in the cytoplasm. The introns are also removed from the pre- mRNA by splicosomes. Introns are part of the primary transcript but do not provide useful information for protein coding. Exons are segments that will code amino acids for proteins.

11 After introns are removed the exons join together to form the final mRNA molecule Some organisms use the mRNA editing to mix up and form new types of mRNA and new proteins. Only eukaryotes perform mRNA processing.


13 IV. The Genetic Code The genetic code – triplets of mRNA nucleotides that determine an amino acid These triplets are called codons There are 64 variations of codons The genetic code is Universal Continuous Redundant (wobble)

14 Start codon – AUG (also determines methionine) – start translation Stop codons – UAA, UGA, UAG – stops translation


16 V. Transfer RNA (tRNA) Clover leaf structure anticodon on clover leaf end amino acid attached on 3 end

17 VI. Ribosomes Small organelles that are made up of rRNA and proteins. Located on the rough ER or in the cytoplasm. Function: makes proteins by reading the code on the mRNA molecule

18 Binding sites: A-site binds to the tRNA anticodon that carries the proper amino acid P-site binds to the last entering tRNA that holds the growing polypeptide chain E-site allows the exit of the empty tRNA

19 VII. Translation Review: protein structure and function Translation – The process of converting the information from the language of nucleic acids to the language of proteins. It takes place in ribosomes in the cytoplasm or on the surface of the endoplasmic reticulum The product of this process is a polypeptide chain (primary protein structure)

20 The actual process is composed of an initiation, elongation, and termination stage. Initiation all participants of translation come together: mRNA (for genetic information) bind to the small subunit of the ribosome. The tRNA molecule that carries Met (start codon) also attaches to the small subunit (on the A-site). Finally, the large subunit binds to the small subunit. The copying starts at the 5 end of the mRNA

21 This process requires energy (GTP provides it)

22 Elongation: The ribosome moves over and reads the next codon on the mRNA. The matching tRNA enters to the A-site, the previous tRNA moves over to the P- site. The amino acids on the two tRNA molecules bind by peptide bond. This process keeps repeating until a stop signal on the mRNA is reached.


24 Termination starts when the stop codon on the mRNA is reached. Instead of the tRNA, a release factor comes in, reads the triplet and makes the entire complex of mRNA, tRNA, polypeptide fall apart.

25 Related animations: html html /student_view0/chapter3/animation__h ow_translation_works.html /student_view0/chapter3/animation__h ow_translation_works.html

26 VIII. Gene Mutations Mutations that only change one single gene by changing the nucleotide sequence of the DNA. Ways of changing the DNA: Substitution Deletion – can cause a change of the reading frame Insertion – can cause a change in the reading frame

27 The result of gene mutations can be the following: Silent mutation – if the change in the DNA sequence does not change the forming polypeptide. Missense mutation – if the change in the DNA sequence changes the type of polypeptide that forms Nonsense mutation – if a stop codon is reached early on and no protein forms

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