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Chapter 12: DNA and RNA. Wheatley-Heckman Honors Biology/Chemistry. DNA. DNA is a long molecule made up of units called nucleotides . Each nucleotide is made up of 3 basic components: 5-carbon sugar (deoxyribose) Phosphate group Nitrogenous base.

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Chapter 12: DNA and RNA

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Chapter 12: DNA and RNA


Honors Biology/Chemistry


  • DNA is a long molecule made up of units called nucleotides.

  • Each nucleotide is made up of 3 basic components:

    • 5-carbon sugar (deoxyribose)

    • Phosphate group

    • Nitrogenous base

  • There are 4 kinds of nitrogenous bases in DNA:

    • Adenine

    • Thymine

    • Cytosine

    • Guanine

  • Two scientists named Watson and Crick concluded that the shape of the DNA molecule was a double helix, in which two strands of DNA were wound around each other.

  • The backbones of the double helix are formed by the sugar and phosphate groups of each nucleotide.

  • The nitrogenous bases stick out sideways from each strand.

  • Years before the double helix was discovered, a scientist named Edwin Chargaff discovered that:

    • The percentages of guanine (G) and cytosine (C) are almost equal in any sample of DNA.

    • The percentages of adenine (A) and thymine (T) are almost equal in any sample of DNA.

  • This became known as Chargaff’s Rules

  • Using Chargaff’s rule, Watson and Crick determined that hydrogen bonds could form only between certain bases, and provide just enough force to hold the 2 strands of the double helix together.

  • The base-pairing rules became:

    • For every A (adenine) on one strand, there is a T (thymine) on the other strand.

    • For every C (cytosine) on one strand, there is a G (guanine) on the other strand.

The two strands are said to be “complementary”.


  • Following the base-pairing rules, write the complementary strand of DNA for the given sequence of nucleotides:


Building a DNA Molecule

  • Black pentagons: Deoxyribose sugar

  • White tubes: Phosphate groups

  • Colored tubes: Nucleotides

    • Orange: Adenine

    • Green: Thymine

    • Blue: Guanine

    • Yellow: Cytosine

  • Build a single nucleotide containing the nitrogenous base adenine (orange).

  • Now build a second nucleotide, but containing cytosine (yellow).

  • Connect it’s phosphate group to the sugar of your first nucleotide. This is forming a sugar-phosphate backbone.

  • Continue adding nucleotides until you have completed a strand following the code below:

    A C T G G A T C T

  • Once that single strand is completed, build a complementary strand.

  • Connect the two strands with hydrogen bonds (white solid pegs).

  • Twist the two strands to make a double helix.

DNA Replication

  • During DNA replication, the DNA molecule separates into two strands, and then produces two new complementary strands following the rules of base pairing.

  • Each strand of the double helix serves as a template, or model for the new strand.

  • Step 1: An enzyme called helicase “unzips” the double helix by breaking the hydrogen bonds between bases.

  • The region where the two strands are splitting is called the replication fork.

  • Step 2: A second enzyme, DNA polymerase, adds complementary nucleotides to each of the original separated strands.

  • As DNA polymerase adds new nucleotides, it also “proofreads” to avoid any errors in base-pairing.

  • Each strand of DNA has a 5’ end and a 3’ end, referring to the orientation of the 5-carbon sugar.

  • The strands run anti-parallel to each other.

  • DNA polymerase can only add to the 3’ end of a DNA strand.

  • Leading strand: strand that reads 5’-3’

    • Replicated continuously.

  • Lagging strand = strand that reads 3’-5’

    • Creates fragments called “Okazaki Fragments” that are later connected by the enzyme ligase

    • This process continues until the entire double helix has been replicated.

    • The process is considered to be semi-conservative because each of the 2 new double helices contains one original strand and one newly synthesized strand.

    Review Animation

    RNA and Protein Synthesis

    • Genes are coded DNA instructions that control the production of proteins.

    • These genetic messages must first be copied from DNA to RNA.

    • The RNA is then used to make proteins.

    Structure of RNA

    • There are 3 main differences between DNA and RNA:

      • The sugar in RNA is ribose instead of deoxyribose.

      • RNA is generally single-stranded.

      • RNA contains uracil in place of thymine.

    Types of RNA

    • There are three main types of RNA:

      • messenger RNA (mRNA)

      • ribosomal RNA (rRNA)

      • transfer RNA (tRNA)

    • Messenger RNA (mRNA)carries copies of instructions for assembling amino acids into proteins.

    Ribosomes are made up of proteins and ribosomal RNA (rRNA).

    • During protein construction, transfer RNA (tRNA) transfers each amino acid to the ribosome.

    Protein Synthesis

    • The process of making proteins has two steps:

      • Transcription

      • Translation

    DNA strand







    • Helicase separates the DNA double helix, and an enzyme called RNA polymerase uses one strand of DNA as a template to make a single strand of mRNA.

    • Regions called promoters determine which segment of DNA (which gene) the RNA polymerase will copy, or transcribe.

    • RNA is formed using the same base-pairing rules discussed before, except that uracil has replaced thymine.

    • “Transcribe” the following DNA strands into mRNA:



    RNA Editing

    • Some DNA within a gene is not needed to produce a protein. These areas are called introns.

    • The DNA sequences that code for proteins are called exons.

    • As the mRNA leaves the nucleus, the introns are cut out of RNA molecules.

    • The exons are the spliced together to form mRNA.

    The Genetic Code

    • Proteins are made by joining amino acids into long chains.

    • There are 20 different amino acids, and it is the unique combination of amino acids in your proteins that determine your genes.

    • Amino acids are coded for by mRNA.

    • A codon consists of three consecutive nucleotides on mRNA that specify a particular amino acid.

    • There are 64 different 3-letter codons that only code for 20 amino acids.

    • This means that more than one codon can code for the same amino acid.

    • Some codons serve as a “stop” signal, and do not code for actual amino acids.

    • Transcribe the following DNA strands into mRNA, and then translate the codons into amino acids using the wheel:

    • DNA Strand 1: TAC GGC AGT

    • DNA Strand 2: GAC TTT CCA


    • Translation is the decoding of an mRNA message into a polypeptide chain (protein).

    • Translation takes place on ribosomes.

    • During translation, the cell uses information from messenger RNA to produce proteins

    • The ribosome binds new tRNA molecules and amino acids as it moves along the mRNA.

    • Each tRNA molecule holds an anticodon that is complementary to a codon.

    • As tRNA brings amino acids to the ribosome, the ribosome will form peptide bonds between the amino acids, forming a chain.

    • This chain will grow until the ribosome reaches a stop codon, which triggers the release of the polypeptide chain.

    • The completed polypeptide chain (protein) will be sent to the golgi body, where it will be packaged and shipped to its final destination.




    Summary of Protein Synthesis

    • Transcription

    • Process by which genetic information encoded in DNA is copied onto messenger RNA

    • Occurs in the nucleus

    • DNA  mRNA

    • Translation

    • Process by which information encoded in mRNA is used to assemble a protein at a ribosome

    • Occurs on a Ribosome

    • mRNA  protein

    Types of Mutations

    • Mutations are changes in the genetic material.

    • Gene mutations: involve changes in one or a few nucleotides, known as point mutations.

      • Substitutions

      • Insertions

      • Deletions

    • Substitutions usually affect no more than a single amino acid.

    • The effects of insertions or deletions are more dramatic.

    • The addition or deletion of a nucleotide causes a shift in the grouping of codons.

    • Changes like these are called frameshift mutations.

    In an insertion, an extra base is inserted into a base sequence.

    In a deletion, the loss of a single base is deleted and the reading frame is shifted.

    • Chromosomal mutations involve changes in the number or structure of chromosomes.

    • Chromosomal mutations include deletions, duplications, inversions, and translocations.

    • Deletions involve the loss of all or part of a chromosome.

    • Duplications produce extra copies of parts of a chromosome.

    • Inversions reverse the direction of parts of chromosomes.

    • Translocations occur when part of one chromosome breaks off and attaches to another.

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