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Watson and Crick
• Watson and Crick studied the work of
others to determine the structure of DNA
• Figured that it is a “Double Helix”:
– Twisted ladder
Chromosomes
“DNA Packaging”
• Chromatin: The substance inside a nucleus.
• Chromosomes: Individual strands of DNA
wound up in a tight package.
What are genes?
• Passed from one generation to the next
• Have to be exactly copied each time
• Contain the information to:
– Build cell parts
– Determine what something looks like
• Found on the chromosomes! (which are
made of DNA)
DNA’s parts
• DNA is made of NUCLEOTIDES
• Each nucleotide has:
– Phosphate
– Sugar
– Base containing nitrogen
4 types of bases
• There are 4 types of bases (see page 208 and
210)
–
–
–
–
Adenine
Thymine
Guanine
Cytosine
• In the 1950s, Chargaff
• discovered that A always
• goes with T, and G with C
The Genetic Code
Were you paying attention?
1. What is the shape of the DNA molecule?
__________________
2. Who figured out the shape of this
molecule?
______________ and ____________
3. Sections of DNA that determine how
something is built or what something
looks like (a trait) are ___________
4. In the nucleus of a cell the DNA is wound up
in tight packages called ____________
5. What is each nucleotide made up of?
___________, ___________ and a
_______________
6. Which base goes with Guanine?
___________ with Thymine? ___________
7. What do small 3 letter sections of the DNA
code for?
________________
How does DNA replicate?
• Replicate = copy
• Watson and Crick also figured out how
DNA replicates!
• Steps to replicate:
– “Double Helix” unwinds
– Ladder unzips
– New nucleotides come in to make two new
strands
Steps to replicate:
making a copy of the DNA
1. “Double Helix” unwinds and unzips
– Enzyme (chemical) called helicase does this
• Next, the exposed bases on each strand
find new nucleotide partners
The end result!
Proteins:
These are what the
information in the DNA is for – to make
the proteins that make the organism.
Protein Synthesis
Making Proteins
• DNA mRNA
ribosome
reads
the codon (3 bases)
tRNA adds
proper amino acid
Transcription
• Transcription is when a strand of mRNA
(messenger RNA) is made from a gene
(section of DNA code).
• The DNA molecule is split down the
middle by an enzyme only at that specific
gene.
• Then new bases come in and line up with
the bases only on one side of the
unzipped section of DNA.
Transcription Cont.
• THE DIFFERENCES BETWEEN
REPLICATION AND TRANSCRIPTION:
– A single stranded mRNA molecule is created
during transcription.
– Uracil matches up with Adenine instead of
Thymine during transcription. The RNA
alphabet is AUCG instead of the ATCG of
DNA
– Instead of the sugar deoxyribose, RNA just
has ribos (which is why it is RNA instead of
DNA)
Protein Synthesis
• After Transcription the mRNA strand can
take the code outside of the nucleus.
• The mRNA codon (piece of mRNA with a
code for making proteins) goes to a
ribosome.
• At the ribosome pieces of tRNA (transfer
RNA) bring amino acids to the ribosome
and match them up with the code on the
codon to make a protein.
• Proteins are just a string of amino acids.
Quiz
1. What is ½ of each rung on the DNA ladder
called?
Nucleotide
2. What does Adenine pair with in DNA?
Thymine
3. What type of RNA takes the code outside
of the nucleus?
mRNA
3. What are genes?
Sections of Chromosomes or DNA
4. What does Adenine match up with in
RNA?
Uracil
5. What are the steps of protein synthesis?
1. mRNA is made.
2. The mRNA stand goes outside the
nucleus to a ribosome.
3. tRNA brings amino acids to piece
together to make a protein.
6. After DNA replication what are we left
with?
Two identical pieces of DNA
7. What part do enzymes play in DNA
replication and transcription?
They unzip the DNA (another kind of
enzyme also zips it back up again).