What is DNA? What does it code for?
base Nucleic Acids Involved in the transfer of genetic information. Two categories: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Both are polymers of nucleotides sugar POLYMERRNA –tens to thousands of basesDNA millions of bases MONOMER “nucleotide”or “base” DNA- A,T,G, CRNA- A,U,G, C
Components of Nucleic Acids (Ch 21.1) The base part of nucleic acids are composed of pyrimidine and purine heterocyclic molecules
* Pyrimidines * * * * * * U T C * Purines Nucleotide Bases * * * * A G A box is where it connect to the sugar
The Sugar and Phosphate 5’ 1’ No OH 3’ Primes refer to sugars in nucleic acids 1’ one prime (attached to base) 3’ three prime (attached to phosphate) 5’ five prime (attached to phosphate)
Nucleotides Nucleic acids like DNA or RNA are a string of nucleotides Nucleotide
Nucleotide or base Base Base Base 3’ 5’ 3’ 5’ 3’ 5’ 3’ Backbone Fig. 21-4, p.652
Primary Structure of DNA Has a sugar – phosphate backbone
Structure of DNA, cont. Double-helix secondary structure of DNA Deduced by Watson and Crick (1953).
Complementary strands of DNA in the double helix are held together by hydrogen bonds.
Telemer Centromer 46 chromosomes in humans (or 23 pairs of chromosomes) Next slide: Size of DNA Chromosomal Packaging of DNA
Size of DNA • Actual size (alpha helix): 2nm wide 1.8 m long • 3.1 billion base pairs • Size of Power cord (5/16”) = 4.4 million miles 180 times around the Earth • Size of thread 150 microns wide = 2,142 miles long • chromosome 1 247M bases = 172 miles • chromosome 22 49k bases = 34 miles
In order for DNA not to get tangled and for it to be accessable it goes through 5 levels of folding The first level is the alpha helix that is the secondary structure in the book. The four other foldings would be considered tertiary
DNA Replication An exact copy of DNA must be made to pass on to new cells of the next generation. A is paired with T, C is paired with G. Semiconservative replication - new DNA molecule which has one strand from the parent and a new complementary strand.
DNA Replication, cont. Step 1: Unwinding of the DNA by helicase and formation of a replication fork. Step 2: Synthesis of DNA segments (Okazaki fragments) by DNA polymerase between replication forks. Step 3: Joining the Okazaki fragments by DNA ligase.
Ribonucleic Acid (RNA) Differs from DNA Has ribose sugar unit instead of deoxyribose Contains the base uracil (U) instead of thymine (T). Usually single stranded.
Three Types of RNA Messenger RNA (mRNA) – carries genetic information from DNA in the nucleus to the site of protein synthesis in the cytoplasm. Ribosomal RNA (rRNA) – main component of ribosomes that are the site of protein synthesis. (Are part of the mRNA reader: (Reads the mRNA, lines up right parts (tRNA), Assembles the protein) Transfer RNA (tRNA) – delivers individual amino acids to the site of protein synthesis.
Are part of the mRNA reader that will Read, Translate and Create a protein from the instructions Letters, hold individual amino acids, fits into the Reader Each unique protein requires one mRNA to code for its synthesis RNA reader, Primary protein structure creator Human rRNA 4 types Small Subunit (1 RNA unit and 30 different proteins) 18s 2300 5.8s 156 Large Subunit (3 RNA units and 45 different proteins) 28s 4200 5s 120 Table 21-1, p.662
Transfer RNA t-RNA See next slide for detail Fig. 21-14, p.663
Make (Create) the protein from the instructions at the factory Copy the code (instructions) The Flow of Genetic Information Genes are segments of DNA that contain information necessary for the synthesis of proteins. Done in the Nucleus Done in the Cytoplasm at the Ribosomes
Transcription – RNA Synthesis Catalyzed by RNA polymerase
Transcription, cont. Transcription produces heterogeneous nuclear RNA (hnRNA) which contains information from both intron and exon DNA segments. Exon DNA segment codes for amino acids. Intron DNA segment carries no code for amino acids. It was originally thought (but this is changing) that these were junk – evolutionary leftovers Three reasons why they are most likely necessary Cells know that they exist and exactly cut them out Some allow for a single gene to code for multiple proteins (alternative splicing, mix and match parts) Some introns have been found to act as controls
Transcription, cont. Segments of hnRNA produced by introns are removed, producing mRNA which then exits the nucleus.
Scan0019 Which means that it is not junk but a designed control
Within a gene that represents a protein, a codon is a three letter code that represents an amino acid or some other instruction Translation – The Genetic Code, cont. There are 43 or 64 combinations of 4 letters in groups of 3.
Holds amino acid amino acid Binding spot Translation - The Structure of tRNA Anticodon (code)
Translation – Protein Synthesis Step 1: Chain Initiation mRNA and a small ribosomal subunit join so the initiating codon (AUG) is aligned with P site of subunit. tRNA brings in methionine (eukaryotes) or N-formylmethionine (prokaryotes). Large ribosomal subunit attaches to complete ribosome.
Translation – Protein Synthesis, cont. Step 2: Chain Elongation The next incoming tRNA bonds at the A site on the mRNA. Peptide bond is formed catalyzed by peptidyl transferase. Translocation of the ribosome releases the “empty” tRNA and makes the A site available to receive the next tRNA.
Human rRNA (the reader/translator) 4 RNA units in 2 subtypes Small Subunit (and 30 different proteins) 18s 2300 5.8s 156 Large Subunit (and 45 different proteins) 28s 4200 5s 120 Table 21-1, p.662
Translation – Protein Synthesis, cont. Step 3: Chain Termination Occurs when ribosome reaches a stop codon. Several ribosomes can be attached to an mRNA strand, called polysomes or polyribosomes.
Mutations Any changes resulting in an incorrect base sequence on DNA. Occur naturally during DNA replication. Induced by environmental factors. Ionizing radiation (X-rays, UV) Mutagens – chemical agents. Not necessarily harmful.
Controls of protein formation Nucleus Old two step model- much too simplified. biofeedback Transport Need outsideCell Need insidecell The Newer model that is more realistic with at least 10 steps and at least 7 control points (and this may still be too simplified) Ribosome Apoprotein Complex multiprotein structures * Site of Activity Transport Transport Outside of cell (somewhere else in organism) * In same cell *
Recombinant DNA and Genetic Engineering Uses restriction enzymes to cleave DNA molecules. Uses DNA ligases to connect DNA fragments. Often uses plasmids as vectors to insert DNA into new hosts producing recombinants.