CHAPTER 2 Notes

CHAPTER 2: Reproduction is the Manifestation of Heredity

Cells are the basic units of life.
The chemical reactions characteristic to life occur in and on cells.
Two major categories of cells exist: prokaryotic and eukaryotic.
- Prokaryotic cells have no nucleus.
- Eukaryotic cells have a nucleus and other membrane bound organelles.
- There are also other major distinquishing differences between prokaryotes and eukaryotes.
- Cell walls of peptidoglycan are found in prokaryotic cells.

Prokaryotes (Bacteria)

These divide by binary fission.
Some species of bacteria can divide as quickly as once every 20 minutes when in conducive growth environments.
Bacterial species exist that can survive in a broad range of environments including the saltiest water, the temperature of boiling water and freezing temperatures.
Some use sulfur in place of oxygen, reduced minerals for energy and/or methane as an energy and carbon source. All require a source of carbon, energy and water. Other elements such as nitrogen, phosphorus and iron are also required.
All require genetic information as does all life.

Eukaryotic Cells

Eukaryotes have or are cells, each which has a nucleus.
RNA is synthesized in the nucleus, except for the small amount done in chloroplast and mitochondria.
The nucleus has a membrane called the nuclear envelop composed of two separate lipid bilayers. The envelop has pores for control of molecular traffic; and, the envelop is continuous with the endoplasmic reticulum.
Eukaryotic cells have other organelles such as the endoplasmic reticulum, golgi, mitochondria and others. See Figure 2.2.
Also, see Figures 2.3a and b.

Chromosomes

Bacteria have a single, continuous circular, double-stranded DNA molecule for a chromosome.
Eukaryotes have linear chromosomes, which are very long compared to prokaryotes, and are associated with an equal mass of protein. See Figures 2.4 and 2.5.
During mitosis and cell division, eukaryotic chromosomes are coated with components of the nucleoli, which will be used to construct the new nucleoli.

Eukaryotic Chromosome Description

A species typically has the same number of chromosomes within them, but generally different numbers between species.
Eukaryotic chromosomes have centromeres, which hold pairs together during mitosis and cell division. See Figure 2.5.
Eukaryotic chromosomes have specialized ends called telomeres.
Eukaryotic chromosomes have other DNA sequence elements such as multiple replication origins, centromeric DNA, telomeric DNA, genes, promoters, enhancers, tandem and nontandem repeats, introns, exons and junk DNA.

Cell Cycle

(Figure 2.6)

A eukaryotic cell begins its existence in the period of the cell cycle called G₁. If it progresses past G₁, the cell enters the S phase when DNA is synthesized.
Next phase is G₂, the periods after S and before M phase.
M phase, mitosis, is when the chromosomes actually separate and then cells divide into two.

M Phase

Many eukaryotes are diploid meaning their chromosomes come in matched pairs.
Exceptions are haploid organisms, like many fungi, which have just one of each chromosome during most of their life cycle.
Humans start out as haploid cells that then merge to form one diploid cell - the DNA molecules then exist in pairs.
With each complete cell cycle, the number of copies double, and then return to diploid after the cell divides.
After replication but before separation, the copies of chromosomes are called chromatids.
During prophase, chromosomes thicken. At conclusion of prophase, the nuclear envelope vesiculates.
During metaphase, chromosomes align on the equator of the cell.
During anaphase the centromeres are pulled apart by attached microtubules and associated proteins. See Figure 2.8.
Telophase is when the chromosome (copies) are at opposite sides of the cell and a furrow forms between them. See Figure 2.7.

Meiosis

Meiosis is similar to mitosis, except the separation of genetic copies occurs twice, so each gamete is haploid. See Figure 2.10.
First, and uniquely to meiosis, homologous copies must pair.
During the first meiosis, the homologous chromosomes separate from each other.
During second meiosis, the sister chromatids separate.
Cells then are haploid.

Cell Check Points

The cell must decide whether it will divide or not.
Halfway decisions are inappropriate for survival. The decision is made by just a couple kinds of molecules and their interactions.
Many factors input to make the final decision to divide.
Many outputs occur to direct the activities that are required prior to and during division.
The two major molecular players are:
- CDK4 and cyclin D for humans.
- A similar two exist in other eukaryotic organisms’ cells.
- Cyclin was discovered by clever scientists who were looking at changes in protein concentrations during cell cycles in early frog and sea urchins embryos.
- Cyclin increases during periods preceding movements between different cell phases particularly before S and before M.
The concentration then drops rapidly and requires time to build back up.
Non-dividing cells, like muscle and nerve cells, do not have cyclin at all, and are said to be permanently at G₀.
The cyclin dependent kinases (CDK) are kinases. A kinase phosphorylates amino acids of certain proteins affecting their activity. Cyclin binding activates CDK's kinase activity.
A complex system composed of these kinds of components are set up to make an all or none decision about cell division. See Figure Cell Cycle Control.

During Meiosis I, Prophase, Homologous Chromosomes Exchange Pieces by Breaking and Reannealing at a Purported Recombination Nodule

The exchange of DNA between homologous chromosomes constitutes a blending of the genetic information the individual got from each of its parents. (Figure 2.11)
This increases variation among progeny.
A limited number of exchanges take place. (Figure 2.12b)

Mistakes During Meiotic Division

Nondisjunction is when chromosomes or chromatids fail to separate or distribute properly during anaphase to telophase.
This can occur during meiosis I or II.
Nondisjunction also can occur during mitosis. It has been associated with some forms of cancer.
Humans normally have 23 pairs of chromosomes.
An example of a mistake made during meiosis is an extra chromosome 21, which is the cause of Down Syndrome.

Egg Maturation is Different

Asymmetric cell divisions occur during meiosis I and II when eggs are forming. See Figure 2.13.
Spermatogenesis rarely results in nondisjunction, but during oogenesis it is much more probable. See Figure 2.14.

Gametes Formation in Plants and Fungi

In plants, the gametophyte is the haploid part of the life cycle and sporaphyte, the diploid part of the life cycle. See Figure 2.15.
In a fungus, for example Neurospora crassa, a bread mold, the haploid state is the major part of life cycle. See Figure 2.16.
- Interesting to geneticists, the order of the spore is the same as the products of each meiotic division.

Zea mays

The major life cycle form is diploid (sporaphyte). See the review of plant sexual organs.
Stamens are the male organ, and these generate diploid spores called microspore mother cells.
Pistils are the female organ, and generate diploid megaspore mother cells.
Microspore mother cells divide twice to form 4 haploid microspores and then differentiate into pollen grains.
In the pistil, the megaspore mother cell undergoes divisions to form four haploid nuclei - one develops further.
- The other three meiotic products degenerate.
- The one that develops divides 2 times producing 8 haploid cells.
- Two fuse to create diploid endosperm
See Figure 2.18 for finer detail.

Drosophila

Very important contributions to genetic understanding come from experimentation using this organism.
Please review the life cycle of the fruit fly. See Figure 2.19.