DNA and Genes Biology: Chapter 11

DNA Deoxyribonucleic Acid (DNA) The genetic material Contains the “blueprint” for an organism Instructions for an organism’s traits Nucleic Acid-polymer (chain) of nucleotides Very long molecule A single strand is ~5 feet long If all uncoiled, your DNA would reach from the Earth to the Sun 600+ times! DNA is too large to leave the nucleus through the pores

DNA, Genes, and Chromosomes DNA=hereditary material Gene=segment of DNA that codes for a specific protein Chromosome=tightly wound strand of DNA and proteins

Structure of Nucleotides A nucleotide is a subunit of DNA (a monomer) 3 parts: A Sugar (deoxyribose) A phosphate group A nitrogenous base (Adenine (A), Thymine (T), Cytosine (C), or Guanine (G) All organisms have these same 4 bases Bases come in 2 forms Purines: Adenine and Guanine Pure as Gold Pyrimidines: Cytosine, Uracil (to be discussed later) and Thymine Py Cut

Structure of DNA DNA is a string of nucleotides The structure was discovered by Rosalind Franklin using X-Ray technology

Watson and Crick James Watson and Francis Crick (1953) “Used” Frankin’s work to come up with the 3-D structure of DNA DNA resembles a twisted ladder or twisted zipper Called a Double Helix Nucleotides are held together by hydrogen bonds

Nucleotide Sequences The sequence of nucleotides determines the make-up of the organism The sequence of nucleotides on a strand of DNA is called a gene A gene is a segment of DNA that is a code for a specific protein

DNA Replication DNA must constantly replicate Simplified Process New copy is used during cell division and production of sperm and egg (mitosis and meiosis-to be discussed later) Simplified Process An enzyme called DNA Helicase breaks the hydrogen bonds between nucleotides This “unzips” the strand Another enzyme called DNA Polymerase adds a complementary nucleotide

Complementary Base Pairing A bonds to T (apple tree) C bonds to G (car garage) Practice: What would be the complementary strand for the following? A T G C T G T A C G A C

RNA Also a nucleic acid (polymer of nucleotides) Very similar to DNA 3 parts: A sugar (ribose) A phosphate group A nitrogenous base (Adenine (A), Cytosine (C), Guanine (G), and Uracil (U) Uracil replaces Thymine Bonds to Adenine 3 types of RNA Messenger RNA (mRNA)-Brings instructions from DNA to cytoplasm (ribosome) Ribosomal RNA (rRNA)-Binds to mRNA and assembles amino acids in correct order Transfer RNA (tRNA)-Carries amino acids to the ribosome

Transcription DNA double helix unwinds RNA nucleotides (A, U, G, C) bond to the complementary base mRNA strand breaks away mRNA leaves the nucleus and travels to the cytoplasm, where it binds to a ribosome rRNA within the ribosome binds to the mRNA and “reads” the code

Amino Acids “The language of proteins uses an alphabet of amino acids.” There are 20 common amino acids The sequence of nucleotides determines the sequence of amino acids 3 bases code for one amino acid This group of 3 bases is called a codon Example: U U U = phenylalanine Several codons code for the same amino acid Example UUU and UUC both code for phenylalanine Some codons are not codes for amino acids, but instructions to start or stop transcription. AUG=Start codon (Methionine) UGA, UAA, and UAG=Stop codons

Translation Translating mRNA into a sequence of amino acids to form a protein Amino acids dissolved in the cytoplasm are brought to the ribosome by tRNA tRNA contains a sequence of 3 amino acids called an anticodon The anticodon is complementary to a codon. Example: AUG is a start codon and codes for the amino acid Methionine Methionine will be attached to a tRNA molecule that has the anticodon UAC As the amino acids attach to the mRNA strand, the tRNA releases and returns to the cytoplasm to pick up another amino acid Peptide bonds are formed between the amino acids, forming a protein

Protein structure The sequence of amino acids determines the protein’s shape Proteins always form in the same 3-D shape These proteins become enzymes and cell structures. Central Dogma of Biology DNARNAProteins This occurs in all organisms It also explain how you get your traits You get some DNA from mom, some from dad, and this DNA determines the proteins and, therefore, all of the traits you have.

When things go wrong Mutation-Change in DNA sequence Can be caused by many different things Errors in DNA replication or transcription Errors in cell division External factors (mutagens) Radiation (sun or tanning) Smoking Alcohol Asbestos Many more

Results of Mutations Most mutation are bad Some mutations are neutral Can result in cell division going out of control Leads to a mass of cells, called a tumor This is how cancer happens. Some mutations are neutral Minor change, or the cell catches it and fixes or destroys it before In rare cases, mutations are beneficial Give the organism something it did not previously have These are what natural selection acts on.

Point Mutations Change in a single base pair in DNA Alters the amino acid, and therefore the entire 3-D structure of the protein

Frameshift Mutations Shifts the reading of codons by one base Unlike point mutations, all amino acids after the mutation are altered, and this drastically changes the protein shape.

Chromosomal Mutations Change to the structure of a chromosome Part of the chromosome may break off, or fold incorrectly

Repairing DNA Mutation sometimes occur and are able to be fixed Certain enzymes are able to “proofread” DNA to ensure that all is well Can replace incorrect nucleotides, or instruct the cell to destroy the DNA containing the errors This process is not perfect, but usually works well Best course of action is avoiding mutagens as much as possible.