Crash Course: Biology

Genetics: Meiosis, Mitosis, etc.

First, we'll define a few terms because that's important.

autosome - any chromosome that's not a sex chromosome

homologous pairs - members of a chromosomal pair [called homologous pairs]

diploid - each chromosome is represented twice [2n] as a member of a homologous pair

haploid - each chromosome is represented once [1n] in an unpaired condition

gamete - a haploid reproductive cell

zygote - the diploid cell resulting from the union of a male haploid gamete and a female haploid gamete

chromatin - the component material of chromosomes, visible as clumps of threads in the nuclei under a microscope.

chromosome - thread-like structures in the nucleus that carry genetic information

chromatid - one of the strands of a divided chromosone, joined by a centromere to its sister chromatic

centromere - a region of a chromosome to which fibers attach to during cell division

A Note on Centromere Position

metacentric = in the middle

submetacentric = close to the middle but not in the middle

acrocentric = very close to one end.

In humans, normally, the higher the chromosome number, the closer the centromere is to one side. (Chromosome 1 is metacentric, but Chromosome 8 is submetacentric and Chromosome 21 is acrocentric).

Interphase is boring, but it has three stages.

It also makes up a good 90% of the cell cycle (figure 1.2). The first stage of Interphase, the G1 [Gap 1] stage,  begins immediately after division. In the G1 stage, the cell merely grows, meaning cytoplasm, organelles, etc are synthesized. No DNA replication is involved. However, the size of the cell doubles. After the G1 stage, there's a G1/S checkpoint to help prevent cancerous growth of cells. Next is the S [Synthesis] stage. Here, DNA is replicated, and the chromosomes become double stranded. After the S stage comes G2 [Gap 2]. Here, the mitochondria divide and precursors of spindle fibers, used in the division, synthesize. After this stage, there is one final checkpoint before cell division to help prevent cancerous growth.

An Aside: If there are checkpoints between the stages, why is cancer still a problem?

The problem lies in two things: proto-oncogenes, which become oncogenes, and the loss of tumor suppressor genes, which break the proto-oncogenes down before they can get to the cell to promote division.

For example, let's take the APC gene for colon cancer. Normal, healthy people are born with two copies of the functioning gene, one from each parent.People with a predisposition to the disease only carry one functioning APC gene. 

Wait, what?

Let's go over this in more detail.

The APC gene is a tumor suppressor gene. It breaks down beta-catenin before the beta-catenin binds to the TCF in the cell and promotes division. Normal, healthy people have two copies of the functioning gene, one from each parent. So, if one gene becomes faulty (through whatever DNA mutation means possible), the other copy of the gene is still functioning, and you're okay. For now. (If the other one breaks, you're screwed).