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.

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.)?

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!

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.

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!

C * G
T * A
A * T
A * T
T * A
G * C
T * A

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

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.

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 to 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.

All organisms use the same code. Thus bacteria can read our genes and produce enzymes (proteins) we need.

Homologous: roughly the same

Sister chromatids: a doubled chromosome

Centromere: center point that holds 2 sister chromatid together.

Centrosomes: cytoskeletal structures that form the centers of the soon to be divided cells

Microtubules: small protein structures that connect to parts of the cell and help pull them to their appropriate sides during cell division.

Interphase

Most of the cell’s life is spent in this phase.

Interphase contains 3 phases:

G1 phase: the first stage in a newborn cell (gap or growth phase)

S phase: DNA duplication takes place.

G2 phase: the 3rd stage in a cell (gap or growth phase), it happens before division

G phases serve as checkpoints.

Mitosis: Step 1, Prophase

2 steps

Step 1: the chromatin condense into compacted chromosomes

Step 2: the nuclear envelope breaks down and microtubules connect the kinetochores to the centrosomes.

Mitosis: Step 2, Metaphase

The microtubules line up the chromosomes at the cells equator

Mitosis: Step 3, Anaphase

The chromatids separate and the new chromosomes move towards the poles.

Mitosis: Step 4, Telophase

The separating chromosomes reach the poles. The nuclear envelopes re-form and the chromatin DEcondenses.

Cytokinesis is taking place during this step.

Cytokinesis

How cells divide

First the cells doubles all its organelles (except nucleus, we just witnessed why).

Next it forms a protein ring around its center at the plasma membrane. This ring begins contracting during telophase and completes shortly after.

Sexual Reproduction

We have to form the gametes.
We break the 46 chromosomes into 2 pairs of 23 in 2 different cells.
Haploid: a cell that only contains one set of chromosomes (ex. Gamete)
Diploid: a cell that contains both sets of chromosomes (ex. Fertilized egg a.k.a. zygote)

Meiosis I : Separating chromosomes

How we create sex cells

Metaphase I: generate chromosome pairs and line them up in the center of the cell

Anaphase I: homologous chromosomes separate

Telophase I & cytokinesis: first cell division resulting in 2 haploid cells

Meiosis II: Cutting chromosomes

Remember: we are working with 2 cells now (we split in meiosis I)
Metaphase II: line up each chromosome in the center of the cell
Anaphase II: pull apart the chromosomes.
Telophase II and cytokinesis: divide into 2 cells each. Results in 4 haploid cells.