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 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
• 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 (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
• Forms a strand of mRNA
• 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.” (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 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
• 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.