Of what are chromosomes composed?

• Proteins (balls) and nucleic acids (long double-stranded strings)

• Nucleic acids of chromosomes are called DNA (deoxyribonucleic acids)

• DNA is spooled around proteins called histones , giving us chromosomes.

DNA

• Remember that Nucleotides consist of a nitrogenous base, a pentose sugar (deoxyribose for DNA and ribose for RNA), and a phosphate group

• Nitrogenous bases are either purines (two rings) or pyrimidines (a single ring)

• Purines include guanine and adenine • Pyrimidines include cytosine, thymine, and

uracil (U is only in RNA) • How did we figure this out?

Watson & Crick

• Used X-ray diffraction data they obtained from Rosalind Franklin.

• They also used Chargaff’s rule: amount of A=T and C=G

• Used tinker toys to find a model that worked with the data

• Watson and Crick were awarded the Nobel Prize in Physiology and Medicine in 1962 for their work. They shared the honor with Maurice Wilkins, the coworker of the then deceased Rosalind Franklin, because the Nobel Prize is never awarded posthumously.

Real Biologists of Genus : Watson & Crick

• We salute you Watson & Crick. As a pair of lonely guys and their tinker toys, you were able to compile the work of others and take it one step further. You found the structure of DNA and you were able to win the Nobel prize.

Chargaff’s Rule

• Erwin Chargaff noticed that the percentage of Adenosine (A) always equaled the percentage of Thymidine (T) and that the percentage of Guanosine (G) always equaled the percentage of Cytidine (C ).

How does DNA code for genes?

• Alphabet with 4 letters   1. A (adenine)   2. T (thymine)   3. C (cytosine)   4. G (guanine)

• A stretch of DNA (gene) codes for proteins (enzymes).

• You have 30,000 genes. Each is a recipe for an enzyme.

• Proteins are strands of amino acids (20 types).

• If each letter (A, T, C, G) represented an amino acid, for how many amino acids could we code?

• Four• But we need to code for

20!• How about if we used 2

letter words (e.g., AA, AT, AC, etc.)?

How DNA codes for proteins

• There would be 16 possible combinations, but we need to code for 20 different amino acids!

AA TA CA GA AT TT CT GT AC TC CC GC AG TG CG GG

• How many combinations are there with 3 letter ‘words’?

• 64!• So that’s how it

works!• Soon, we’ll practice

translating proteins.

• DNA (deoxyribonucleic acid) is a double strand of nucleotide bases wound around itself like yarn. 

• The two strands of DNA are complementary, as a base in one strand bonds to the base across from it. 

• There are different four types of bases: A (adenine), T (thymine), G (guanine), and C (cytosine).  A always bonds to T, and G always bonds to C. 

• Thus if you know one strand, you can determine the other strand.

Let’s try it

C

T

A

A

T

G

T

• G

• A

• T

• T

• A

• C

• A

Remember:

•Genes are stretches of DNA that code for an enzyme.

•Each enzyme helps make a chemical reaction occur in your body.

•That’s how genes affect the way you are!

How genes code for us

• Why are identical twins basically identical? 

• Because they have the same genes. 

• Any differences you can see between them are termed ‘environmental’ as opposed to ‘genetic’.

Why are traits linked?

• Remember that we only have 23 pairs of chromosomes and thousands of genes.

• It’s a space issue. Each chromosome codes for thousands of different genes.

• Genetic recombination: the cross-over of genetic material for one chromosome to its respective paired chromosome (ex. A piece of one 21 trades with a piece of another 21)

DNA never leaves the protection of the nucleus

• DNA is protected by the nucleus.

• If it left the nucleus and got destroyed or altered, the cell would be different and every cell created from this cell would have the same defect or mutation.

• Thus we create proteins from genes by first making a copy of the gene.

• That copy is called RNA.

• Then the RNA leaves the nucleus to be translated into a protein.

Mutation and Cancer

• Mutation: an alteration of the genetic code

• Tumor: a solid cell mass formed by the inappropriate proliferation of cells

• Benign tumor: a tumor that remains confined to one area

• Malignant tumor: a tumor that is able to send out cells to proliferate somewhere else in the body

Mutation

• neutral mutation: If just one nucleotide is copied incorrectly and it makes no difference to the amino acid chain.

• point mutation: If just one nucleotide is copied incorrectly and it makes a major difference to the amino acid chain.

DNA RNA protein

Cell

nucleus DNA

RNATranslation: RNA protein

Transcription: DNA RNA

RNAribosome

Transcription takes place in the nucleus

Translation takes place at the ribosome.

Roles of RNA:

• mRNA: messenger RNA is a copy of the DNA to be translated. The mRNA is transcribed from DNA and then travels outside the nucleus to the ribosome.

• rRNA: ribosomal RNA is the main machinery that accomplishes translation by reading the mRNA and getting the appropriate amino acid (the building block of proteins) from tRNA.

• tRNA: transfer RNA is set to grab a particular amino acid based on its label. The rRNA reads the label and knows that the appropriate amino acid is attached to the tRNA. Don’t worry about tRNA for the exam.

• RNA does serve as the genetic storage in some viruses that do not have DNA.

Transcription: DNA RNA

• Transcription takes place in the nucleus.

• Scribes used to copy books before the printing press.

• RNA is like DNA except it has U (uracil) instead of T.

• GATTACA (DNA)• CUAAUGU (RNA)

• Before the gene can be transcribed, the double-stranded DNA molecule must be unwound a bit so the two strands can be separated. Then the RNA bases are matched to the DNA strand to complete transcription.

• Now let’s try translation, where we translate from nucleic acid language to amino acid language.

RNA translation

• Once an mRNA molecule reaches the ribosome, it can be translated into a protein.

• The ribosome (constructed of rRNA) grabs the mRNA and reads the molecule three nucleotide bases at a time. Each set of three nucleotides is called a codon.

• Each codon codes for a specific amino acid, using a table.

• For the exam, you should be able to transcribe DNA to RNA and then translate the RNA to an amino acid sequence (i.e., a protein) using a Table provided.

Translation Code

What is GGATTACCAATG?

The universal code of life

• All organisms use the same code.

• Thus bacteria can read our genes and produce enzymes (proteins) we need.

• Ex. Insulin. 1. Used to harvested from corpses or pigs 2. Now we get bacteria to make it for us.