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Concept 2: Central Dogma, Replication, and Reading DNA

Concept 2: Central Dogma, Replication, and Reading DNA. Biology Standard 4.3 and 4.4 Sections 8.3-8.5. DNA Replication. As we learned during cells, in order for a cell to successfully divide it must complete the cell cycle, and progress through an “S” or “synthesis” stage.

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Concept 2: Central Dogma, Replication, and Reading DNA

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  1. Concept 2: Central Dogma, Replication, and Reading DNA Biology Standard 4.3 and 4.4 Sections 8.3-8.5

  2. DNA Replication • As we learned during cells, in order for a cell to successfully divide it must complete the cell cycle, and progress through an “S” or “synthesis” stage. • During this stage a copy of DNA is made. • For the daughter cells of a cell division to be identical, each must have a copy of the DNA from the original cell. • The process of DNA replication ensures that each new cell has identical DNA.

  3. DNA Replication • Recall that DNA is a double helix and consists of two chains. • Each chain contains a very specific sequence of nucleotides that are complementary to the other strand.

  4. DNA Replication • Replication begins by unzipping the two strands from one another. • Each side of the unzipped molecule can now serve as a template for making a copy. • Nucleotides that are floating free in the nucleus will begin to bond to their exposed complementary bases.

  5. DNA Replication

  6. DNA Replication • Enzymes will now bind and speed up the rate that complementary bases bind to one another. • The end result is two identical DNA molecules.

  7. DNA Replication • The end result is two identical DNA molecules.

  8. Central Dogma of Biology • So one of DNA’s jobs is to make more of itself through replication. However, it has a second job as well. • DNA must also be able to make, or at least instruct a cell how to make something else as well, namely proteins. • Discovery of this knowledge led to what is known today as the Central Dogma of biology. That is, • DNA  transcription  RNA  translation  protein

  9. Central Dogma of Biology

  10. Reading Genetic Code • Within a gene, DNA is read in groups of three nucleotides. • Thus, the entire genetic language is read by reading “words” that are only three letters long. • For example, take a look at the section of DNA nucleotides below. • TACAGAACCCACATT • This “sentence” is broken down into the following three-letter words when read by the cell. • TAC AGA ACC CAC ATT

  11. Reading Genetic Code • Each of these triplets in the genetic code stands for a specific amino acid. • Recall these are the monomer subunits of proteins, or polypeptides, that we learned about when studying macromolecules.

  12. Transcription • The first step in the Central Dogma involves transcribing DNA into messenger RNA, or mRNA. • Remember what nucleotide is replaced in RNA?

  13. Translation • The second step involves interpreting the mRNA, called translation, and placing amino acids in the correct sequence. • Each triplet corresponds to a specific amino acid. • Take the following as a “sample sentence”, starting with DNA. • TAC AGA ACC CAC ATT • This would transcribe as the following mRNA codons • AUG UCU UGG GUG UAA • This would translate as the following sequence of AA’s • Methionine Serine Tryptophan Valine STOP

  14. Transcribed m Translated

  15. Protein Synthesis • When a particular protein is needed, the body goes through protein synthesis. • The information for a protein is carried in DNA, which resides in the nucleus, but DNA itself does not leave the nucleus.

  16. Protein Synthesis • Protein synthesis is carried out in the cytoplasm, by ribosomes. • Thus, the DNA code must be carried from the nucleus to the cytoplasm. • This is accomplished by mRNA, or messenger RNA. m

  17. Protein Synthesis: Translation • Once mRNA is attached to a ribosome, another type of RNA, called tRNA, will begin bringing over appropriate amino acids. • tRNA stands for transfer RNA, because its job is to transfer amino acids to the ribosome. tRNA Ribosome

  18. Protein Synthesis: Translation • Ribosomes hold the mRNA in place, while tRNAs bring over the amino acids coded for on the mRNA. • Look at your decoding wheels, and you will see that AUG codes for Methionine (our START). Attached amino acids Ribosome tRNA tRNA mRNA

  19. Protein Synthesis: Translation • Additional tRNAs are brought over and position themselves accordingly. • As they do so, the amino acids they carry align, and the ribosome forms a peptide bond between the amino acids.

  20. Protein Synthesis: Translation • Eventually, a STOP codon will be reached, and protein synthesis will terminate. • The chain of amino acids produced is the protein.

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