Chapter 12 DNA and RNA
Discovery of DNA How do genes work? Several scientists from began investigating the chemical nature of genes. DNA was determined to be the molecule responsible for coding all genes.
What is DNA? Deoxyribonucleic acid (DNA) is a long molecule made up of units called nucleotides. Nucleotides are made up of: 5-carbon sugar (deoxyribose) Phosphate group Nitrogenous base (adenine, guanine, cytosine, or thymine) All the genetic information for an organism is carried on its DNA.
Nucleotides Form DNA Nucleotide Structure
Structure of DNA DNA wounds around itself, forming a twisted ladder (double helix) Discovered in 1950s by Rosalind Franklin using X-ray diffraction. “Double Helix” term coined by Watson and Crick in the 1950s as well.
Groups of Bases Purines (larger) Adenine Guanine Pyrimidines (smaller) Cytosine Thymine
Structure of DNA Cont’d Chargaff’s Rules: The four bases (adenine, guanine, thymine, cytosine) pair up in a certain way A – T C – G The bases are held together by hydrogen bonds (hydrogen bonds are weak bonds)
Storage of DNA Prokaryotes – DNA is found in the cytoplasm Most have single circular DNA molecule Eukaryotes – Much more DNA than in prokaryotes (up to 1000x as much!) Condensed into chromosomes Chromosome number varies wildly from one species to the next. (humans have 46)
DNA Replication Before a cell divides, it must copy its DNA in a process called replication. During DNA replication, DNA separates into two strands. DNA polymerase makes complimentary strands using base pairing. Each strand of the original DNA molecule is used as a template for the new DNA.
The area where DNA unwinds is called a replication fork.
RNA RNA is ribonucleic acid. Like DNA: Made up of nucleotides Codes for proteins Unlike DNA: Nucleotides are made of ribose (instead of deoxyribose) RNA is single-stranded (DNA is double) The base Thymine is replaced with Uracil in RNA
3 Types of RNA Messenger RNA (mRNA) RNA that carries copies of DNA instructions Ribosomal RNA (rRNA) Make up ribosomes (along with proteins) Transfer RNA (tRNA) Transfers or carries each amino acid to the ribosomes as coded for by the mRNA
Transcription RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complementary sequence of RNA DNA strands are separated, RNA polymerase uses one strand of DNA as a template and assembles RNA Uses base pairing to assemble RNA A – U (instead of T) C - G
Genetic Code The “language” of mRNA is sometimes called the genetic code. The genetic code is read 3 letters (or bases) at a time, called codons. A codon is made up of 3 nucleotides that specify for a single amino acid Amino acids are strung together to form proteins (polypeptides)
Genetic Code, cont. Ex: UCGCACGGU Would be read 3 bases at a time… UCG-CAC-GGU Using the code → Serine-Histadine-Glycine
Start and Stop Codons AUG codon codes for the assembly of amino acids to begin. It also codes for methionine Three codons code for the assembly of amino acids to stop – UAA, UAG, and UGA
Protein Synthesis (Translation) Proteins are assembled on ribosomes using information from mRNA Steps: 1 – mRNA transcribed 2 – mRNA attaches to ribosome. tRNA matches anticodons to codons and brings in amino acids. 3 – amino acids are joined together by peptide bonds. Free tRNA molecules float away. 4 – polypeptide chain grows until stop codon.
12-4 Mutations Gene mutations: Point mutations – involve changes in one or a few nucleotides Substitutions Insertions Deletions Frameshift mutations – shift the “reading frame” of the genetic message Ex: AUG UGG CCU UAC → AUG UGG ACC UUA C
Point Mutations DNA: TAC G C A TGG AAT mRNA: AUG CGU ACC UUA Amino Acids: Met-Arg-Thr-Leu SUBSTITUTION DNA: TAC G T A TGG AAT mRNA: AUG CAU ACC UUA Amino Acids: Met-His-Thr-Leu DNA: TAC GCA TGG AAT mRNA: AUG CGU ACC UUA Amino Acids: Met-Arg-Thr-Leu INSERTION DNA: TA T CGC ATG GAA T mRNA: AUA GCG UAC CUU A Amino Acids: Ile-Ala-Tyr-Leu
Chromosomal Mutations Deletions Duplications Inversions Translocations