Chapter 12
MOLECULAR GENETICS
THE STRUCTURE OF DNA AND
DNA REPLICATION
DNA - DEOXYRIBONUCLEIC ACID
To understand genetics, scientists had to learn the
chemical makeup of the gene.
 Scientists discovered that genes are made of DNA
 Scientists also found that DNA stores and
transmits the genetic information from one
generation of an organism to the next.
 Scientists began studying DNA structure to find
out how it carries info, decides traits, and
replicates itself.

DNA – THE STRUCTURE
DNA is a long molecule made up of units called
nucleotides.
 Each nucleotide contains a 5-carbon sugar, a
phosphate group, and a nitrogen-containing
base.
 There are four kinds of bases in DNA:

 Adenine
(A)
 Guanine (G)
 Cytosine (C)
 Thymine (T)
THE NUCLEOTIDES
Green = 5-C sugar
Peach = phosphate
WATSON AND CRICK
Watson and Crick made a 3-D model of DNA.
 Their model was a double helix, in which two
strands are wound around each other.
 A double helix is like a twisted ladder

 Sugar
and phosphates make up the side of the
ladder
 Hydrogen bonds between the bases hold the
strands together (the rungs)
DNA
CHARGAFF’S RULE
Aka the “base pairing rule”
 Chargaff’s rule states that bonds only form
between certain base pairs

A
always pairs with T
G always pairs with C
You must memorize this!!!!
BASE-PAIRING
WHERE CAN YOU FIND DNA?
Most prokaryotes have one large DNA molecule
floating in the cytoplasm.
 Eukaryotes have DNA in the nucleus.
 Amazing fact:

 The
nucleus of a human cell contains more than 1
meter of DNA!
CHROMOSOMES
Contain both DNA and protein, which are tightly
packed together to form chromatin.
 Chromatin consists of DNA that is tightly coiled
around proteins called histones.
 This forms a beadlike structure called a
nucleosome.
 Nucleosomes pack with one another to form a
thick fiber, which is shortened by a system of loops
and coils.
 Nucleosomes allow enormous amounts of DNA to
fit into such a small region.

DNA REPLICATION
Before a cell divides, it copies its DNA in a
process called replication.
 During replication:

 The
DNA molecule separates into 2 strands – each
strand of the DNA molecule serves as a model for
the new strand.
 Following the rules for base pairing, new bases are
added to each strand.
 The end result is two identical strands.
HOW DOES REPLICATION OCCUR?
Its carried out by a series of enzymes
 Some enzymes “unzip” the molecule of DNA – the
hydrogen bonds between bases are broken
 The two strands unwind and serve as templates to
create two new strands
 The key enzyme is called DNA polymerase – joins
individual nucleotides to produce a DNA molecule


DNA polymerase is also involved in “proofreading” each
of the new DNA strands to minimize errors.
DNA REPLICATION
LET’S REPLICATE!
Original Strand = T A C G T T
New Strand =
AT G CAA
Original Strand = ATCGGCAATCACGAT
TAGCCGTTAGTGCTA
New Stand =
RNA AND
PROTEIN SYNTHESIS
“THE PROTEIN SYNTHESIS FAIRYTALE”

Once upon a time in a land far, far away, there lived a
beautiful but greedy queen named Queen Alicia. The
Queen was good to the people who lived in her kingdom
but she had something, something of great value to her
and she would not share it with anyone. She had locked
up in the tower of the nucleus a magical cookbook and
this cookbook was filled with amazing recipes. The
village was a happy one except for one problem. All they
had to eat was mutton (mushy, fatty meat). One day a
man named mRNA found out that the Queen was hiding
the magic cookbook and he wanted to know the recipes.
He found a way in through a hole called a nuclear pore.
He crawled up to the very highest tower and found the
cookbook. He started to transcribe the information
when Queen Jen entered the chamber.
He ran out through another nuclear pore and ran
through the fields of cytoplasm. He had a problem
though. He could not cook, but he knew who could. He
had heard of this amazing, very cool restaurant called
the Ribosomal Café. The cook was notorious. He could
even make mutton taste good. His name is rRNA.
mRNA knew that RRNA was the man for the job. mRNA
could do the recipes justice, but there was another
problem. The cook only spoke French and mRNA only
spoke English. So he had to find someone he could
trust. He located the man named tRNA. tRNA was a
translator, and could speak both English and French. He
gave the recipes to tRNA and tRNA took them into the
Ribosomal Café and gave them to chef rRNA who turned
them into the most amazing food anyone in the village
has ever eaten. After that you could always hear the
chef singing “le poison, le poison, he he he, hu hu hu”
and everyone lived happily ever after eating fish
(protein).
RNA – THE BASICS
For a gene to work, the genetic instructions in
the DNA molecule must be decoded.
 The first step is to copy the DNA sequence into
RNA.
 RNA is a molecule which contains instructions
for making proteins.

RNA – THE STRUCTURE

RNA is similar to DNA, except for:
 RNA
contains the sugar ribose
 DNA
 RNA
is single-stranded
 DNA
 RNA
has deoxyribose
is double stranded
has uracil in place of thymine – A, U, C, G
 DNA
has A, T, C, G
RNA MOLECULES – WHAT DO THEY DO?

There are 3 kinds of RNA –
 Messenger
RNA (mRNA)
is this in the story? Cookbook
 Has the instructions for joining amino acids to protein
 What
 Ribosomal
RNA (rRNA)
is this in the story? Cook
 Assembles the proteins
 What
 Transfer
 What
RNA (tRNA)
is this in the story? Translator
 Carries each amino acid to the ribosome according to the
coded message in mRNA
 Contains an anticodon to pair up with mRNA
TRANSCRIPTION
Process of making (or copying) RNA from DNA
 Occurs in the nucleus
 During transcription:

DNA strands are separated
 RNA strand is built using one DNA strand as a template
 DNA is transcribed (written) into RNA following basepairing rules except that U binds to A

 AU,
TA
DNA Strand =
TACGCTACGCCTAATACT
New mRNA Strand =AUGCGAUGCGGAUUAUGA
TRANSCRIPTION TO TRANSLATION
The directions for making proteins are in the
order of the four nitrogenous bases
 This code is read 3 letters at a time

 Each
grouping of 3 letters is called a codon
 AUG/CGA/UGC/GGA/UUA/UGA
Each codon stands for one amino acid
 There is one “start” codon and 3 “stop” codons

 Start
= AUG
 Stop = UGA, UAA, UAG
TRANSLATION
Process in which the cell uses info from mRNA to
make proteins
 Takes place in the ribosome
 How does it work?

mRNA moves into the cytoplasm and attaches to
ribosome
 As each codon of the mRNA moves through the
ribosome, the proper amino acid is brought into the
ribosome by tRNA.
 The ribosome joins together each amino acid and the
protein chain grows.
 When the ribosome reaches a stop codon, the
polypeptide chain is released.

HOW DO WE FIGURE OUT THE AMINO ACID?

Use a Codon
Chart
LET’S DO AN EXAMPLE!
1.
2.
3.
4.
DNA Strand =
mRNA Strand =
TACGCTACGCCTAATACT
AUGCGAUGCGGAUUAUGA
AUG/CGA/UGC/GGA/UUA/UGA
Split into codons =
Create Protein = Methionine - Arginine - Cysteine Gylcine - Leucine
MUTATIONS
Mutations are mistakes made when cells copy
their own DNA.
 Mutations are changes in the genetic material
of a cell.
 There are two types of mutations:

 Gene
mutations – changes in a single gene
 Chromosomal mutations – change in the number or
structure of chromosomes and affect multiple
genes
GENE MUTATIONS

Point mutations (substitution)–
 Occur
at a single point in the DNA sequence
 Causes one base to replace another, which only
affects one amino acid

Frameshift mutations (insertion, deletion) –
A
base is added or removed
 All the codons are affected/changed resulting in a
completely different protein
CHROMOSOME MUTATIONS
Change in structure of chromosomes
 Four types:

 Deletions
= ABC-DEF  AC-DEF
 Duplications = ABC-DEF  ABBC-DEF
 Inversions = ABC-DEF  AED-CBF
 Translocations = ABC-DEF  ABC-JKL
GH-IJKL
GH-IDEF