DNA and Genes Biology Chapter 11 DNA Deoxyribonucleic

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 (monomers) • Very long molecule • A single strand is ~3 meters long • If all uncoiled, your DNA would reach from the Earth to the Sun

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 (histones)

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 (based on structure) • 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 • New copy is used during cell division and production of sperm and egg (mitosis and meiosis) • 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 (m. RNA)-Brings instructions from DNA to cytoplasm (ribosome) • Ribosomal RNA (r. RNA)-Binds to m. RNA and assembles amino acids in correct order • Transfer RNA (t. RNA)-Carries amino acids to the ribosome

Transcription • DNA double helix unwinds • RNA nucleotides (A, U, G, C) bond to the complementary base using RNA polymerase • Forms a strand of m. RNA • m. RNA strand breaks away • m. RNA leaves the nucleus and travels to the cytoplasm, where it binds to a ribosome • r. RNA within the ribosome binds to the m. RNA and “reads” the code

Translation • Translating m. RNA into a sequence of amino acids to form a protein • Amino acids dissolved in the cytoplasm are brought to the ribosome by t. RNA • t. RNA 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 t. RNA molecule that has the anticodon UAC • As the amino acids attach to the m. RNA strand, the t. RNA releases and returns to the cytoplasm to pick up another amino acid • Peptide bonds are formed between the amino acids, forming a protein

Amino Acids • “The language of proteins uses an alphabet of amino acids. ” (your textbook) • 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 may 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

Protein structure • The sequence of amino acids determines the protein’s shape • Proteins of a certain sequence 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 • 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.