Meiosis

Meiosis is a process of cell division that takes place only in sex organs, in order to produce sperm or egg cells. Sperm or egg cells must have half the number of chromosomes of normal, somatic cells, so that when they fuse during fertilization, the result is a zygote with the complete number of chromosomes. For example, human somatic cells have 46 chromosomes (diploid), whereas human sperm or egg cells have 23 chromosomes (haploid). To achieve this halving of chromosome number, meiosis is required.

Like in mitosis, cells are in interphase before meiosis begins, during which DNA replication occurs. Thus meiosis begins with DNA in the form of sister chromatids (see The difference between chromatin, chromatid and chromosome). Meiosis then takes places in two major phases: meiosis I and meiosis II.

Meiosis I
At the start of meiosis I (prophase I), homologous chromosomes find each other and pair up in the cell. They then exchange bits of DNA segments, a process called crossing over. Crossing over allows for greater genetic variation, as the resulting chromosome will have some genes from the maternal chromosome and some genes from the paternal homologous chromosome. At metaphase I, the homologous chromosome pairs line up in the equator of the spindle. Then, at anaphase I, the pairs are separated and moved to opposite poles. The cell then undergoes cytokinesis to divide into two daughter cells, the chromosomes decondense into chromatin strands, and the nuclear membrane reforms around the chromatin (telophase I). This means that at the end of meiosis I, the resulting cells will already have half the amount of chromosomes. However, a second phase is needed to separate the sister chromatids that form each chromosome.

Meiosis II
Meiosis II is very similar to mitosis. The chromosomes (sister chromatids) are separated into two individual chromatid strands so that there are 23 chromatids per daughter cell.

Thus at the end of meiosis, 4 haploid cells (gametes) are produced.

This image from the Molecular Biology of the Cell (Alberts, Garland Science 2002) shows clearly the steps...

The difference between mitosis and meiosis

Mitosis is the process of cell division where one cell divides into two cells which are identical to the original cell. This takes place in all our somatic ("normal") cells.

Meiosis is a special form of cell division that only takes place in the sex organs (gonads) of organisms that undergo sexual reproduction, in order to produce sperm or eggs (gametes). Here, the resulting sperm/egg cells are not identical to the original dividing cell, but only have half the amount of chromosomes as our normal cells (haploid).

Human somatic cells normally have 46 chromosomes; 23 of them come from the mother, and the other 23 from the father. These somatic cells are referred to as being diploid, meaning they have 2 copies of each chromosome. Meiosis is basically a halving process so that the sperm or egg will finally end up with 23 chromosomes each (haploid), so that when they fuse during fertilization, the result is 46 chromosomes again.

Human somatic cells have 2 copies of 23 chromosomes (one copy from the mother, and one copy from the father), totalling 46 chromosomes

Read more about the process of mitosis and meiosis.

The difference between chromatin, chromatid and chromosome

It's easy to confuse these 3 terms! Let's try to clear things up here.

DNA, the blueprint of life, is organized into structures called chromosomes. In prokaryotic cells, chromosomes are circular, whereas in eukaryotic cells, they are linear strands. Different organisms have different numbers of chromosomes: human cells usually have 46 chromosomes, dogs have 78 chromosomes, while kangaroos have only 12 chromosomes!

This karyotype of a human male cell shows the 46 chromosomes.

When you add all these chromosomes up, each cell actually contains about 2m of DNA! And all this DNA has to fit into a tiny nucleus of 5-10um in diameter. This is like trying to stuff a piece of string 2km long (it will take you about 20 minutes to walk from one end to the other) into a tiny bead smaller than 1cm!!! To do this seemingly impossible feat, cells devised an ingenious packaging system: it wraps DNA around proteins called histones. The resulting DNA-protein complex is called chromatin.


Chromatin consists of DNA wrapped around histones

At the beginning of cell division (S-phase), the DNA is replicated, producing two identical copies of DNA, which are connected to each other at the centromere. This replicated X-like structure is now called a sister chromatid pair. A chromatid is therefore just one of the strands.

During mitosis, the sister chromatid pair condenses further, giving rise to the fat X chromosomes that you can see in the karyotype above.

Therefore, chromosomes can be found in 3 forms: thread-like chromatin (during G1 of interphase), thread-like sister chromatids (during S-phase of interphase) and the condensed, visible form (during mitosis).
difference chromosome chromatid chromatin
When a cell divides, the sister chromatids separate, and each daughter cell receives one of the strands. The chromatid then decondenses into a long single chromatin strand when the new cell goes into interphase.

For more information about DNA, check out this Scitable entry: DNA Is a Structure That Encodes Biological Information

Photosynthesis Lab: What to Expect

As mentioned earlier, the rate of photosynthesis should increase when you give the plant more light, more carbon dioxide and the optimum temperature. However, at high light and high carbon dioxide, photosynthesis will reach its maximum rate and won't go any higher.

For more details, check this page on photosynthesis: http://click4biology.info/c4b/3/Chem3.8.htm
(I will eventually get around to writing more about photosynthesis on this website).


Other Important Points


Here are some extra notes about photosynthesis that could come in handy when discussing your results and writing your conclusion and evaluation sections.

  • Plants carry out respiration (the opposite of photosynthesis!) the entire time to make energy, and so they constantly USE oxygen and MAKE carbon dioxide (which lowers the pH). At low photosynthesis rates (e.g. in the dark), they are respiring but not photosynthesizing. You will only start to see bubbles or pH changes when the rate of photosynthesis is higher than the rate of respiration.

  • Having different number of leaves or mass of the plant cutting will affect your results. More leaves mean more photosynthesis! Also, they should all be healthy cuttings.

  • Remember to take your measurements only after a few minutes of exposing your plant to light so that it will be photosynthesizing at a constant rate. Similarly, be careful not to expose all your plant cuttings to light while running an experiment on one of them!

  • If you are using a pH probe, it needs to be properly calibrated before starting your experiments. If you are not able to do this, just remember that it might be a source of error...

Try to make a list of all the things you will need to run the experiment. Think about what you need to keep constant. You should also have a control experiment where photosynthesis won't happen (no source of light or carbon dioxide, or a dead plant, for example).

Read more about setting up your experiment, and what variables you can measure.

Photosynthesis Lab: What to Measure?

What to Measure?


Now, how would you measure the rate of photosynthesis?

Again, let's look at the equation for photosynthesis:

CO2 + H2O + light --> O2 + glucose

There are 2 things we can quickly measure in this experiment (the dependent variables): amount of oxygen produced, or, amount of carbon dioxide used.

If the rate of photosynthesis increases, the rate of oxygen production goes up, and the rate of CO2 consumption rises too! If the rate of photosynthesis goes down, then we can expect the opposite effect: oxygen production drops and carbon dioxide is not used as quickly.

But how can we measure oxygen or carbon dioxide levels?

It's quite easy with Elodea!

Measuring Oxygen


In water, oxygen that is produced by the Elodea cutting is released as bubbles from its leaves. The rate of oxygen produced can be measured by either counting the number of bubbles released in a certain amount of time (bubbles/min), or by trapping the oxygen gas in an inverted syringe or tube and measuring the volume of oxygen produced in a certain amount of time (cm3/min).

It's important that you give the plant a few minutes to photosynthesize before starting your measurements. This ensures that the plant is making oxygen at a constant rate. (You should also check this visually, before starting to count the bubbles). To have more accurate results, take the measurement at least 3 times. You could also vary the amount of time used to count the bubbles (10 seconds, 30 seconds, 1 minute). Then calculate the number of bubbles per minute or per second.

Measuring Carbon Dioxide


To measure carbon dioxide levels, we can measure the pH of the water. This is because when carbon dioxide dissolves in water, it forms carbonic acid and lowers the water pH. Therefore, in the opposite situation -- i.e. when carbon dioxide is used -- the pH goes up.

CO2 + H2O --> H2CO3 <--> H+ + HCO3-

pH probe by Sergei Golyshev (http://www.flickr.com/photos/29225114@N08/)
pH probe by Sergei Golyshev (http://www.flickr.com/photos/29225114@N08/)


As the changes in pH are probably quite small, it would be necessary to use a pH meter (or pH probe) with a digital readout. pH paper may not be sensitive enough to detect the changes. Measure the pH once before starting the experiment. Then, start taking pH measurements after the plant is consistently producing oxygen bubbles. Take a few measurements at 5 minute intervals (for example, you can run the experiment for 15 minutes and that would give you 3 readings).

Remember to rinse the pH probe with distilled water before transferring it to your test tube. You should also make sure that the test tube water is well-mixed before taking your measurement, by stirring with a glass rod, or inverting the tube a few times (make sure you use a rubber stopper so you don't spill it all!).

Another important note: make sure you use really clean test tubes, plants and stoppers! Any contaminants could affect your pH reading. Rinse everything with tap water before you start your experiment!

Read more about setting up your experiment, and what you should expect and look out for.

Photosynthesis Lab: Set Up

Elodea by Kristian Peters (http://www.flickr.com/photos/fabelfroh/)
Photo of Elodea plant by Kristian Peters (http://www.flickr.com/photos/fabelfroh/) 

A commonly-used plant for such experiments is the Elodea, an aquatic (underwater) plant also referred to as pondweed. You can buy this at aquarium stores.

The set up depends on which question you would like to study.

In general, the plant is placed in a test tube filled with diluted sodium bicarbonate solution (1%). Sodium bicarbonate (baking soda) gives the plant a source of carbon dioxide so that they photosynthesize more quickly.

You need to provide the plant with a fixed light source (a light bulb, for example). You could place a large beaker (around 2 liters) filled with water between the light source and the test tube so that the light heats up only the beaker water and not the plant!

For your experiment, you should change only ONE variable, never more than that (if you can help it!).

Light Intensity


Take the case of investigating the effects of light intensity on photosynthesis rates. To change this, you can either use different wattage light bulbs, add screens between the light bulb and the plant, or change the distance of the light bulb from the plant. If you want to be really accurate, get a light meter and measure the light intensities that the plant is receiving with the different settings.

from The Biology Web (http://faculty.clintoncc.suny.edu/faculty/michael.gregory/)
from The Biology Web (http://faculty.clintoncc.suny.edu/faculty/michael.gregory/)

Carbon Dioxide


To change carbon dioxide levels, just change the concentration of sodium bicarbonate in the solution (e.g. 0.5%, 1%, 1.5%, 2%).

Temperature


To change the temperature, you could warm the plant/test tube gently using a water bath method, i.e. place the test tube containing the plant in another large beaker which is filled with cool or warm water of known temperature. Place a thermometer in the test tube so that you know when the set temperature has been reached, and to make sure that it doesn't change too much during the experiment.

Read more about what variables you can measure, and what you should expect and look out for.

Photosynthesis Lab

Here are some ideas for planning experiments for studying plant photosynthesis.

Remember that the simple equation for photosynthesis is:

CO2 + H2O + light --> O2 + glucose

Elodea by Kristian Peters (http://www.flickr.com/photos/fabelfroh/)
Photo of Elodea plant by Kristian Peters (http://www.flickr.com/photos/fabelfroh/)
There are many questions you can ask about photosynthesis, such as:

  • Does light intensity affect the rate of photosynthesis?

  • Does temperature affect the rate?

  • What about levels of carbon dioxide made available to the plant?

For your experiment, you can change what you provide the plant with (the independent variable) and see how this affects their ability to photosynthesize.

Read more about setting up your experiment, what variables you can measure, and what you should expect and look out for.

Hope the explanations here will help you write and plan your next lab report!

Feel free to send me more comments, suggestions and feedback!