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 SynthesisMaking 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.

Quiz1. 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?

6. mRNA is made.

7. The mRNA stand goes outside the nucleus to a ribosome.

8. 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).