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5A. Genes, Chromosomes, and Cell Division
Transcript of 5A. Genes, Chromosomes, and Cell Division
Think about your family members: Father, Mother, Brothers, and/or sisters. Think about their appearance and how it compares to you. You may realize that each members of the family look alike but at the same time there are features that are also different. This is because there genes that you share between yourselves which give similar qualities but not exact.
- characteristics that every memeber of a species possess. (eg. all humans / or all dogs)
- other features that makes an individual unique.
Nature vs. Nurture: Argument
If genetics decide the characteristics of an organism, then is the fate of all living organisms written in the DNA? yes and no.
Genes do determine the species characteristics and individual characteristics. But the extent to which these characteristics will be expressed depends in many ways upon the environment.
5.1 The Mechanism of Heredity
- study of heredity.
Aristotle - proposed that offspring was a mixture of elements from the parent's blood.
Preformationists - there are tiny, completely formed organisms in the sperm that, when planted in an egg, will grow up.
Gregor Mendel - proposed there are pairs of factors in organisms and that each parentgives a single set of these factors to its offspring.
The study of genetics begin with:
- section of DNA that produces a particular polypeptide chain of amino acids, that causes a particular trait.
This is true for both macro and micro aspects of an organism.
It's also important keep in mind that not all genes are used for the structuring of protein, some genes are used as markers for activation and deactivation of protein-coding genes.
DNA is efficiently and tightly comprsessed to store all the information.
Each strand of DNA consists of many genes stringed together in a chain. These long strains of genes are called
When the cells are not dividing, chromosomes appear as a fuzzy, tangled mass.
Some of the genes are actve, producing RNA
Others are not used and are just kept and stored
- one of the two DNA duplicates that compose one chromasome. (When chromosome is not duplicated)
- region that holds the two chromatds together, until ready to separate.
Humans have 46 chromosomes in total. Species of different kind may have different number ofspecies.
It should not be confused, that the number of chromosomes dictates the type of species. It is the genes that give organisms its traits.
- An illustraion in which the chromosomes of a cell are arranged according to their size.
- Chromosomes that are in pairs.
- When referring to a member of a homologous pair.
- Cells with homologous pairs of chromosomes.
- Cells that have half the number of chromosomes as diploid cells.
Both homogues in a pair have genes for the same charcteristics. Meaning that humans have 2 of almost every gene.
The Cell Cycle
Cells undergo cyclic events throughout its life time, that has various influences on the cell itself. These events can be divided into 3 major stages:
Though the stages are divided up, it does not necessarily mean that they must occur one after the other, the event can/may overlap one another.
Most of the time in a cell cycle is spent during the interphase. Interphase can further be divided into G1, S and G2 phases.
G1 - Cell undergoes major growth spurt
increase in size, new organelles, proteins + other molecules
S - DNA is duplicated
centrosome divides - produce fibers to be used in separating process later on.
G2 - Produces proteins and molecules needed for mitosis
- The division of the nuclear material so that each new nucleus has a complete identical copy of the genetic information from the mother cell. Consists of 4 phases.
Cytoplasm: centrosomes migrate to opposite poles of the nucleus.
Mitotic Spindle: special microtubules that "direct" the movement of the chromosomes during mitosis.
Nucleus: Chromosomes get short and thick as they coil up
Nuclear membrane disintegrates, nucleolus disappears.
- stage when the centromeres are aligned on the equatorial plane of the cell.
sister chromatids appear to repel each other, forming the familiar X shape of a chromasome.
enzymes break down proteins in the centromeres, allowing the two chromatids to separate.
considered daughter chromasomes
Spindle fibers pull chromasomes towards opposite poles of cell.
Phase begins when the new chromosomes reach the ends of the spindle.
A new nucleus begins to re-form around the chromosomes , forming two daughter nuclei.
chromosomes begin to uncoil and the nucleoli reappear.
Mitotic spindle begins to disappear
Once the duplicated chromosomes have been separate into two sets, the cytoplasmic contents (organelles, proteins, membranes, and cytoskeleton) are divided.
contractile ring begins to divide the cell. The contractile ring tightens which causes the plasma membrane to pinch in. eventually the cell divides and form 2 daughter cells.
small membrane-bound vesicles (formed by golgi apparatus) align and fuse forming a membrane-bound cell plate.
later cellulose and other materials are created forming cell walls.
Variations in Mitosis
Some unicellular organisms will go through mitosis but not cytokinesis.
Result in single cell with multiple nuclei.
The length of time for division may differ between different types of cells.
Some cells may undergo cell division every 10 minutes.
There may be variations in the length of time of cell phases.
result in phases starting early or late.
Functions of a cell may determine the rate of cell division.
eg. skin cells must constantly divide to replace dead or washed-off cells.
eg. bone cells much slower compared to skin cells
Some cells once reaching maturity, does not divide anymore.
eg. Scientists - brain, nerve cells - does not divide, instead grow larger to replace dead cells.
Uses of Mitosis
As you may be aware by now, essentially, cell division is the process of making 1 cell into 2 cells with the same DNA. The most obvious use of mitosis is:
However, some organisms also use mitosis as a means of reproduction. This form of reproduction is called
. This form of reproduction reproduces offspring that are identical to the parents. There are 3 ways this can take place.
These organisms are capable of breaking a part of themselves off, and the broken part will grow up to be the new species.
These organisms produce new small organism on the side of the parent.
The organisms release their cells protected by hard coverings, to be grown elsewhere.
Scientist (Edouard van Beneden) - noticed that reproductive cells (sperm and egg/ova) had only half the number of chromosomes (Haploid cells) compared to mature cells (Diploid cells).
meaning that there was a some sort of process that reduced the number of chromosome in reproductive cells and when they united it restored the total number of chromosome.
- is the reduction of a cell's chromosome number from diploid o haploid through two consecutive cell divisions.
first division -->
second division -->
Each phase still undergo
Metaphase, Anaphase, Telophase
, with a slight variation in some of the phase compared to mitosis.
Genes & Chromosomes
Meiosis I: First Division
chromosomes tightly coil up + mitotic spindle are formed.
Duplicated homologous chromosomes pair up to form a structure called
how is this different from mitosis?
The chromosomes are very close
may result in exchange of genetic information between one another.
Occurs similarly to mitosis, with slight difference in the
Tetrads line up on the equatorial plane of cell.
Rather than the sister chromatids separating (mitosis), homologous pair separate.
Sister chromatids remain together.
Homologues travel to the ends of spindle.
Chromosomes arrive at the poles of the poles of the cell.
the chromosomes do not uncoil
Cytokinesis begins -> the two new cells enter directly into second division of meiosis
Meiosis II: Second Division
At the start of meiosis II, each daughter cell has a haploid chromosome number.
The chromosomes are still duplicated
Meiosis II will produce 4 daughter cells, each with one unduplicated chromosome of each pair.
Mitotic spindle re-forms and the spindle fibers begin to move the chromosomes toward the equatorial plane of cell.
The chromosomes are aligned on the equatorial plane. Compare this stage to metaphase I. Note that there are only one member of the homologous chromosome pair on the equatorial plane.
The sister chromatids separate, and the resulting daughter chromosomes move toward opposite poles.
The nuclei are reformed and each of the four new cells has a haploid chromosome number.
Comparison of Mitosis and Meiosis
The main difference between the two processes is the tetrad formation during the metaphase I stage.
- A haploid cell which can unite with another to from a
The process of forming a zygote (union of gametes) is called
- organisms that produce gametes that are all alike.
- when organisms create gametes in the form of sperm (male) and ovum (female).
- forming of sperm
produces four functional gametes
Often, the cells formed by meiosis lose most of their cytoplasm and form a flagellum
Flagellum propels the DNA towards the ovum.
- forming of an ovum.
During cytokinesis of meiosis I -> One of the two cells receive most of the cytoplasm
Smaller cell - First Polar Body
During cytokinesis of meiosis II in the Larger cell -> One of the two cells receive most of the cytoplasm
Smaller cell - Second Polar Body
Results in 1 Ovum + 3 polar bodies
Polar bodies disintegrate, Large quantity of cytoplasm used for the development of zygote.
- union of haploid gametes, resulting in a diploid zygote.
results in offspring that are not genetically identical to either parent.
have one chromosome of every pair from each parent.
Now then, we know that we obtain half of our DNA from each parent, what decides which gene to be used?