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Double stranded Sugar = deoxyribose Thymine (no Uracil) Stays in nucleus One type Same copy in the cell all the time Single stranded Sugar = ribose Uracil (instead of Thymine) Nucleus & cytoplasm 3 types (mRNA, tRNA, rRNA) Disposable copies DNA vs. RNA
RNA • many functions but mostly just protein synthesis • three main types of RNA: messenger RNA, ribosomal RNA, and transfer RNA
mRNA • RNA molecules that carry copies of the DNA instructions = mRNA • messenger RNA (mRNA) = serve as “messengers” from DNA to the rest of the cell
rRNA • Ribosomes are made up of several dozen proteins, as well as a form of RNA known as ribosomal RNA (rRNA).
tRNA • During the construction of a protein, a third type of RNA molecule transfers each amino acid to the ribosome • as specified by coded messages in mRNA. • These RNA molecules are known as transfer RNA (tRNA).
Transcription (DNA mRNA) • RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complementary sequence in RNA • required enzyme = RNA polymerase • RNA polymerase binds to DNA (in nucleus) • separates the DNA strands • RNA polymerase then uses one strand of DNA as a template • nucleotides are assembled into a strand of mRNA • Transcription Animation
Where does RNA start? • enzyme will bind only to regions of DNA known as promoters • promoters are signals in DNA that indicate to the enzyme where to bind to make RNA (“start sequence”). • Similar signals in DNA cause transcription to stop when the new RNA molecule is completed.
RNA Editing • Intron = intervening sequence of DNA; does not code for a protein • Exon = expressed sequence of DNA; codes for a protein • When RNA molecules are formed, both the introns and the exons are copied from the DNA • introns are cut out of RNA molecules while they are still in the nucleus • exons are then spliced back together to form the final mRNA
The Genetic Code • Proteins = long chains of amino acids (polypeptides) • polypeptide = combination of any or all of the 20 different amino acids • properties of proteins are determined by the order in which different amino acids are joined together to produce polypeptides
The “language” of mRNA instructions is called the genetic code • RNA contains four different bases: A, U, C, and G • Letters read “3” at a time = codon • Codon = a group of three nucleotides on messenger RNA that specify a particular amino acid.
Translation (mRNA tRNA amino acid chain) • Occurs at the Ribosome • mRNA = instructions for the order of the amino acid sequence • Ribosome = reads the instructions of the mRNA
Steps of Translation • mRNA is released from the nucleus enters cytoplasm • mRNA attaches to the ribosome • mRNA codons move through the ribosome proper amino acid brought by tRNA • Amino acids are bound together polypeptide chain
Each tRNA carries only one type of amino acid • The three bases on tRNA = anticodon (complementary to mRNA)
The ribosome forms peptide bonds between the neighboring amino acids • It also breaks the bonds between tRNA and the amino acids • Translation ends when a “stop” codon is reached
What amino acids are made? • Use the mRNA strand on the Genetic Code Chart • DNA: TAC AAA CAC GGA CCA ACT (antisense strand) • mRNA: AUG UUU GUG CCU GGU UGA • tRNA: UAC AAA CAC GGA CCA ACU • Amino acids: Methionine – Phenylalanine – Valine - Proline - Glycine - STOP