DNA Structure and Composition

• Chapters 12 and 13 cover DNA, RNA, and Protein Synthesis

12.2 Structure of DNA

• Erwin Chargaff found that the number of Adenines (A's) is the same as Thymines (T's), and Cytosines (C's) is the same as Guanines (G's)
• Rosalind Franklin's X-ray photo showed DNA has a double helix structure
• Watson and Crick determined that DNA is a double helix, with A's matching T's and C's matching G's
• The discovery regarding DNA structure was on Feb 28th, 1953
• DNA structure was published on April 25th, 1953

DNA Composition

• A nucleotide has three parts: deoxyribose sugar, phosphate group, and a nitrogen base
• Bases are located on the 'inside' of the DNA structure
• Sugars and phosphates are located on the 'outside', forming the backbone
• Hydrogen bonds hold bases together
• Covalent bonds exist between deoxyribose sugar and a phosphate group

Base Pairing

• Purines have 2 rings
• Examples of purines: Adenine (A) and Guanine (G)
• Pyrimidines have 1 ring
• Examples of pyrimidines: Thymine (T) and Cytosine (C)
• Each base-pair contains one purine and one pyrimidine

Double Helix

• DNA is antiparallel, each strand runs in opposite directions
• The direction is based on where the phosphate is attached to the sugar
  • 3' ---> 5'
  • 5' <--- 3'

12.3 DNA Replication

• Enzymes help in the DNA replication process
• Helicase breaks bonds between bases and forms a replication fork
• DNA polymerase adds new nucleotides, in the 5' --> 3' direction
• Ligase glues together the sugars and phosphates using covalent bonds
• Semi-conservative replication results in one original strand (template) and one new strand

Ch 13 PROTEIN SYNTHESIS

• Transcription: DNA is transferred into mRNA
• Translation: mRNA is transferred into protein
• In prokaryotes, transcription and translation both occur in the cytoplasm
• No mRNA processing occurs in prokaryotes
• In eukaryotes, transcription occurs in the nucleus and involves mRNA processing
• Translation occurs on ribosomes, specifically on the rough endoplasmic reticulum (ER)

13.1 RNA

• RNA is single stranded
• RNA contains Uracil (U) instead of Thymine (T)
• RNA has ribose sugar

Types of RNA

• rRNA (ribosomal) makes up the ribosome
• tRNA (transfer) transfers messages from mRNA and carries amino acids to mRNA
• mRNA (messenger) goes to the ribosome and allows the code to get 'turned into' proteins

Genetic Code

• DNA is transcribed into mRNA in the 5' --> 3' direction
• mRNA is 'read' in sequences of 3 bases, known as a codon
• One codon encodes for one amino acid

13.1 Stages of Transcription

• Three stages of transcription: initiation, elongation, and termination
• Initiation occurs at the promoter region, where RNA polymerase attaches
• RNA polymerase slides along the template DNA strand
• Elongation involves the addition of new RNA bases in the 5' -> 3' direction
• Termination occurs when a termination signal indicates to STOP

13.2 steps of translation (protein synthesis)

• Initiation involves tRNA with Methionine (MET) attaching to a small ribosomal subunit
• The large ribosomal subunit then attaches so tRNA is at the P-site
• mRNA is read in the 5' ---> 3' direction
• Elongation involves tRNA bringing the next amino acid to line up with the mRNA codon at the A-site
• A peptide bond forms between the two amino acids, and mRNA moves over
• The tRNA's move over and a new codon is read
• Termination occurs when a STOP codon is reached (UAG, UAA, UGA)
• The polypeptide is freed from the ribosome and goes to the Golgi Body

13.3 Mutations

• Chromosome and gene mutations are two types of mutations

Types of Chromosome Mutations

• Deletion is when part of the chromosome is deleted and is usually harmful
• Duplication occurs when part of the chromosome gets duplicated
• Inversion is when a section of the chromosome gets 'flipped'
• Translocation is when part of one chromosome attaches to another and is usually harmful

Gene Mutations

• Point mutations are a change in one or a few nucleotides
• Substitution is when one base is changed to a different base
• Substitutions can affect a single amino acid, but they don't have an effect
  • This known as a "silent mutation" because the amino acid isn't changed (ex. CUU and CUC both make LEUCINE)
• Insertion/deletion is when one base is inserted or deleted which can cause drastic effects. This is known as 'frameshift mutations' and shifts the way codons are read
• An example of a disease due to mutation is sickle cell anemia