Lecture 1 (Dent)

What does the Cell Theory explain?

One mother cell divides into two daughter cells. For cell division to be a productive use of the cell’s time, both daughter cells have to inherit everything they need to survive from the mother cells, including the information of what properties the cell should have (the stuff of heredity)

What is a Karyotype?

A common way of looking at chromosomes. The karyotype is just a way of staining and organizing chromosomes that makes patterns in their structure more apparent.

Rules for the Karyotype

1) the number of chromosomes is always the same for a given species but varies greatly from one species to another (humans have 46 chromosomes)

2) In eukaryotes, chromosomes come in identical pairs called homologs (humans have 22 homologs and 2 sex chromosomes)

3) At cell division, each chromosome has been duplicated exactly once - these are called chromatids held together by a protein called a centromere (one pair of chromatids goes to each daughter cells)

How are you able to see DNA? And what is chromatin?

During cell division, the DNA condenses around a set of proteins called histones that help keep the long strands tightly packed which makes it visible

The combination of DNA and proteins is called chromatin

Why do cells need a very organized system when it comes to each daughter cells getting exactly one copy of every chromosome?

Because it would never happen correctly by random chance. If each chromatid randomly went to one of the two daughter cells, each pair would have a 50% chance of being split correctly. BUT with 46 chromosomes the chance that all of them splitting correctly is not very likely

What extra chromosome do you have when you have Down syndrome?

Chromosome 21

Do organisms need more than one or exactly one of each chromosome?

exactly one of each chromosome

what are the steps of the cell cycle?

S phase: DNA is replicated

Gap 2: Cell is preparing to divide

M phase: Mitosis - the process by which somatic cells make identical copies of themselves by creating daughter cells that inherit one copy of each chromosome

OR

Meiosis - the process by which germ cells can make non-identical copies of themselves by creating daughter cells that inherit one copy of each homolog - daughter cell ends up with half the DNA of the mother cell

Gap 1: When the cell is resting

Cytokinesis is not represented because it is somewhat optional but normally occurs right after mitosis

Time spent in each stage can vary significantly among cells in the same organism

Interphase refers to all phases except the M phase

Why do we need checkpoints?

to make sure that everything happens sequentially and takes place in the right order to preserve cell function

they contain special checkpoints at the end of each phase before the next phase can begin

For example adding a drug that prevents DNA replication causes the cell to get stuck in S phase and does not go to gap 2

However if a checkpoint is inactivated and the drug is still there, the cell doesn’t know any better and will proceed to the next step but will die because the chromosomes have not duplicated correctly in S phase

Why is the G1 to S checkpoint critical?

It is tightly controlled so that things in the body, like growth happen correctly. If this control system breaks, cells can divide uncontrollably = cancer