DNA
 The common thread
of life.
What are the functions of DNA?
 There are two main functions of DNA
– It is the molecule of heredity.
– It contains the information needed to construct proteins.
What is heredity?
 Heredity is the term used to explain the
transfer of genetic information from one
generation to the next.
 You inherited half of your DNA (your genes)
from Mom, and the other half from Dad.
 DNA is the molecule that allows this to
happen.
Structure of DNA
 As you surely
remember the basic
building blocks of
DNA are
nucleotides.
 Here we see the
phosphate group
and 5-Carbon
Sugar.
Nitrogen bases.
 The third part of a nucleotide is the nitrogen
base.
 There are two types of nitrogen bases:
Purines: A and G which have a double ring
structure.
Pyrimidines: T and C which have a single ring.
Genes to Proteins.
 Your DNA is housed in the nucleus of every cell in
your body.
 Most of the time, the DNA is in long twisted
strands called Chromatin. Very thin, and cannot
be seen using a regular compound light
microscope.
 The Chromatin condenses to form Chromosomes
prior to cell division. These can be seen.
Genes to proteins cont.
 These strands of chromatin are made up of many
genes. A gene can be hundreds or thousands of
nucleotides long. (The entire human genome
consists of 3 BILLION nucleotides).
 Each gene is a series of nucleotides which
contains the information to make a protein.
 1 gene = 1 protein.
The genetic code
 Genetists (Scientists who
study genetics) were
interested in determining how
information in your genes was
used to create proteins.
 They knew that there were 20
different amino acids and that
there were 4 types of
nucleotide.
Cracking the code.
 Since there were twenty amino acids
and only 4 nucleotide types, they knew
that the genetic code had to consist of
more than one nucleotide.
41= 4
Ex. The nucleotide Adenine by itself
does not code of the amino acid
Threonine.
Cracking the code (cont.)
 By applying the same logic, they realized
that the genetic code could not consist of
two nucleotides either.
42=16
Adenine and Cytosine together does not
code for the amino acid Threonine.
Cracking the code
 Expanding on the theme, they realized that sequences of
3 nucleotides would give more than enough combinations
to code for all 20 amino acids.
43=64
Adenine, Cytosine and Guanine DO code for the amino acid
Threonine.
They call these 3-nucleotide sequences CODONS.
We will be using the actual genetic code later in this unit.
The Genetic Code
Lets take a look.
DNA genetic code
DNA Replication
 Prior to cell division (mitosis or meiosis) the
cell must make another copy of, or replicate
it’s DNA.
 The DNA molecule basically unzips itself by
breaking the hydrogen bonds holding the
two strands of nucleotides together.
 Each strand then forms a second strand by
using free nucleotides which are found in
the nucleus.
 DNA replication simple
 DNA replication complex
DNA and RNA
 Both are nucleic acids, and are very similar
to each other.
 There are some differences.
DNA
RNA
Sugar:
Deoxyribose
Ribose
Strands:
2
1
Nitrogen Base: Thymine
Uracil
Location
Nucleus
Nucleus &Cytoplasm
RNA transcription
 While DNA contains an entire
library’s worth of information, it is a
big, big molecule.
 It is so big, it cannot fit through the
pores of the nuclear membrane.
 To get the information needed to
make proteins to the ribosomes,
the DNA needs some help.
Enter the messenger
 In order to get the information stored in a
gene to the ribosome, messenger RNA
(mRNA) is transcripted.
 First, the DNA will partially unzip itself to
expose one gene’s worth of code.
 An enzyme called RNA polymerase then
copies a strand of RNA based on the
template strand of the DNA.
mRNA transcription
 RNA transcription simple
 RNA transcription complex