DNA- The Genetic Material
Why should I care about DNA?
•DNA stores the information that tells
cells which proteins to make
•Your proteins give you your
physical/chemical/ identity
•Your DNA is different from
everyone else’s DNA on earth
Does DNA have anything to do with
why I am so special?
•Yes, it’s the variability of DNA
that makes us all different!
Who Discovered the Structure
of DNA?
James Watson & Francis Crick discovered the structure of DNA
after observing an X-ray picture of it taken by Rosalind Franklin
The Structure of DNA
DNA is composed of 2 connected nucleotide
chains
Sugar-Phosphate backbone, with bases in the
middle
4 Bases
Adenine Guanine
Cytosine Thymine
Bases complement one another
A bonds to T, G bonds to C
(bases bond together by hydrogen bonds)
The two strands are twisted together
Structure of DNA
Additional DNA Facts
•Shape of DNA is a Double Helix (a Twisted Ladder)
•Sugar in DNA = Deoxyribose
•1 base + 1 phosphate + 1 sugar = Nucleotide
Nucleotide
Deoxyribose
DNA Replication
Why does DNA replicate?
•Cells get worn out, new cells must be made
•New cells must make exact copies of themselves
•Copies of replicated genes can be passed on to
offspring
How is DNA Replicated?
1.
Helicase breaks hydrogen bonds in the middle of the
strand, creating a replication fork
2.
Unpaired bases form new bonds with free nucleotides in
the cell
3.
New strand is rewound together by DNA Polymerase,
creating 2 identical DNA molecules
4.
Sometimes replication errors make a mistake (A bonds to
a G);
DNA polymerase usually proofreads the strand
There are 3 x 109 bases in the average cell
Takes about 4 hours for a cell to replicate its DNA
DNA Replication
DNA Replication
Transcription
DNA is the material used to make proteins
DNA is located in the nucleus of the cell
Proteins are made in structures called ribosomes,
in the cytoplasm of the cell
DNA CANNOT leave the nucleus of the cell
because it is double stranded
Transcription
Mr. DelPrato I am afraid for the DNA, it can’t get to
the ribosome! What I am to do?
Relax, RNA is the key!
What is RNA?
•A nucleic acid that is similar to DNA
•Ribose is the sugar
•Instead of thymine, Uracil
•Single stranded
Steps of Transcription
1. DNA strand unwinds (helicase)
2. Free nucleotides bond to bases, except
U in RNA bonds to A from DNA
3. RNA polymerase joins single stranded
RNA together
4.Messenger RNA (mRNA) moves to the
ribosome in the cytoplasm (or on the
Rough ER)
Transcription
2 Models of Transcription
Transcription
Animation of Transcription!
http://www.johnkyrk.com/DNAtranscription.html
Translation (Protein Synthesis)
What is protein synthesis?
The Conversion of RNA to Amino Acids,
which make up proteins
Why is it called translation?
The genetic code (ATCG) is translated
into a protein
Where does it occur?
At the ribosome in the cytoplasm
Globular Protein found in Human
Blood (serum albumin)
How Does it Happen?
1. Initiation
A. Ribosome attaches to mRNA (each 3 bases is called a codon)
B. tRNA brings anticodon and AA to ribosome
2. Elongation
A. More tRNA’s bring more AA’s to ribosome
B. AA’s connected together to make polypeptides (by peptide bonds)
3. Termination
A. Ribosome reaches a STOP signal on mRNA
B. Polypeptide (protein) is broken off and becomes part of the human
body
C. Golgi apparatus modifies completed protein; ER transports it in/out
of cell
Translation- The Movie
Translation Animation Part 2!
Translation Animation
Scary Translation
Scary Animation of Translation!
http://www.youtube.com/watch?v=983lhh2
0rGY
The processes of transcription
and translation are the same in
all living organisms!
Blue Whale
Tarantula
Control of Gene Expression in Eukaryotes
• In eukaryotes, cells differ in which genes are being expressed
based on cell function – ex. nerve vs. muscle.
• Genes in eukaryotic cells are turned on and off like a light
switch. The genes that are turned on in a muscle cell are
different than the genes that are turned on in a nerve cell.
• Gene expression is what makes cells different in a
multicellular organism
Nerve Cell vs.
Muscle Cell
Gene Mutations
• Definition - a change in the sequence of bases within
a gene
Causes –
• Mutations can be spontaneous or caused by
environmental influences called mutagens.
• Mutagens include radiation (X-rays, UV radiation),
and organic chemicals (in cigarette smoke and
pesticides).
Types of Mutations
Frameshift mutations –
•one or more bases are inserted or deleted from a
sequence of DNA
•can result in nonfunctional proteins
•can result in no protein at all – stop codon where
there shouldn’t be one
Point mutations (3 Types) –
•One base is substituted for another
•May result in change of amino acid sequence
•May not affect protein at all
Types of point mutations
1. Silent mutation - the change in the codon results in the
same amino acid
Ex: UAU  UAC both code for tyrosine
2. Nonsense mutation - a codon is changed to a stop
codon; resulting protein may be too short to function
Ex: UAC  UAG (a stop codon)
3. Missense mutation - involves the substitution of a different
amino acid, the result may be a protein that cannot reach its
final shape
Ex: Hbs which causes sickle-cell disease
Repair of Mutations
DNA polymerase proofreads the new strand against
the old strand and detects mismatched pairs,
reducing mistakes to one in a billion nucleotide pairs
replicated.
If errors occur in sex cells – mutation may be passed
onto offspring
If errors occur in body cells - cancer may result