13.1: RNA & Transcription

DNA DNA contains genes (sequences of nucleotide bases) These genes code for proteins Proteins are used to build cells and do much of the work inside cells

Genes & Proteins Proteins are made of amino acids linked together by peptide bonds 20 different amino acids exist long chains of amino acids are known as polypeptides

DNA is found inside the nucleus Proteins, however, are made in the cytoplasm of cells by organelles called ribosomes

DNA‘s code must be copied and taken to the cytoplasm In the cytoplasm, this code must be read so amino acids can be assembled to make proteins This process is accomplished by RNA (ribonucleic acid)

RNA Differs from DNA RNA has a sugar ribose, not deoxyribose RNA contains the base uracil (U), not thymine (T) RNA molecule is single-stranded, not double-stranded

Like DNA, RNA is a polymer of nucleotides Like DNA, RNA is a polymer of nucleotides. In an RNA nucleotide, the sugar ribose is attached to a phosphate molecule and to a base, either G, U, A, or C. Notice that in RNA, the base uracil replaces thymine as one of the pyrimidine bases. RNA is single-stranded, whereas DNA is double-stranded.

Types of RNA

messenger RNA (mRNA): carries information from DNA to the ribosomes ribosomal RNA (rRNA) and other proteins make up ribosomes transfer RNA (tRNA) molecules transfer each amino acid to the ribosome as it builds the protein

DNA  RNA  Protein Eukaryotic Cell DNA Pre-mRNA mRNA Ribosome Protein Nuclear membrane Transcription RNA Processing Translation DNA Pre-mRNA mRNA Ribosome Protein Eukaryotic Cell 10

Transcription & Translation Protein synthesis is the process of making a protein by taking the DNA instructions in the nucleus to the ribosome where the protein is put together This process is accomplished in two phases: Transcription & Translation

Messenger RNA (mRNA) long straight chain of nucleotides made in the nucleus copies DNA & leaves through nuclear pores contains the nitrogen bases A, G, C, U (no T)

Transcription process of copying the sequence of one strand of DNA uses the enzyme RNA polymerase RNA polymerase binds to the gene’s promoter then unwinds & separates the DNA

RNA polymerase then uses one strand of DNA as a template to add nucleotides into a complementary strand of pre-mRNA

mRNA replaces T with U (uracil) making it clear to the cell that the mRNA is a copy

Editing Pre-mRNA After the DNA is transcribed into pre-mRNA, editing must be done to the nucleotide chain to make the mRNA functional Introns, non-functional segments of DNA, are snipped out of the chain by an enzyme

Exons, segments of DNA that code for proteins, are then “spliced” back together before the mRNA leaves the nucleus.

Once the mRNA strand is functional, it leaves through nuclear pores into the cytoplasm