Chapter 12: DNA and RNA

Wheatley-HeckmanHonors 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

• 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”.

Practice!• Following the base-pairing rules, write

the complementary strand of DNA for the given sequence of nucleotides:

• AAGCCAGATAGT

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(template)

TRANSCRIPTION

TRANSLATION

mRNA

Protein

Transcription• 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:

– AACTGACTTC

– GGATCCATCG

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• 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.

mRNA

DNA

Protein

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

Summary of Protein Synthesis

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.