meiosis II. After this cell division
(similar to mitosis and that does not alter ploidy) the cells are still haploid (they have become haploid after meiosis I).
40. How many cells are made
after meiosis I and meiosis II?
After meiosis I two cells with already separated homologous are created. After meiosis II four cells are created.
41. What are the periods of
the first meiotic division?
Meiosis I is divided into prophase I, metaphase I, anaphase I and telophase I.
42. In which period of meiosis
does the pairing of
homologous chromosomes
occur?
The pairing of homologous
chromosomes is a vital step for meiosis because the rightness of the
homologous separation depends on the process. This event occurs in prophase I of the cell division.
43. What is crossing over? In
which period of meiosis does
this event occur?
Crossing over is the eventual exchange of chromosomal fragments between homologous chromosomes. The
phenomenon occurs in prophase I when homologous chromosomes are paired. Crossing over is of great importance for evolution and biodiversity since it
provides recombination of alleles (of different genes) linked in the same chromosome during cell divison by meiosis.
44. What are the “chiasms” of
homologous chromosomes
seen in prophase I?
Chiasms are intersections of two tracts in the form of X.
The chiasms seen in prophase I are chromosome arms crossing over same arms of their homologous. In fact when chiasms are seen under the microscope chromatids are exchanging
chromosomal segments with other chromatids of its homologous.
45. Is there interphase again
between meiosis I and
meiosis II?
There is no interphase nor DNA duplication between the divisions of meiosis. Only a short interval called diakinesis occurs.
46. What are the periods of
the second meiotic division?
Meiosis II is divided into prophase II, metaphase II, anaphase II and telofase II.
47. What are the respective
functions of the separation of
homologous chromosomes
and of the separation of
identical chromatids in
meiosis?
The separation of homologous
chromosomes in meiosis I has two main functions: to reduce to a half the total number of chromosomes, generating haploid daughter cells at the end of the process, and to make possible genetic recombination since the separation is aleatory, i.e., each pair of daughter cells can be different from the other pair relating chromosomal combination from paternal and maternal origins. (And if crossing over is considered each of the four resulting cells can be different from the others.)
The separation of identical chromatids in meiosis II has the same function it has in mitosis: to separate the
chromosomes already duplicated to the daughter cells.
48. During which meiosis
division does ploidy reduction
occur? Does ploidy reduction
occur in mitosis?
In the cell division by meiosis ploidy reduction occurs in meiosis II. Initially, taking as example a 2n somatic cell, ploidy increases to 4n (duplication of DNA) during interphase. During meiosis I, since homologous chromosomes are separated, ploidy falls to 2n (the original number) and then during meiosis II ploidy finally falls to n in the resulting daughter cells.
Ploidy reduction does not occur in mitosis. This fact shows that, although in meiosis ploidy is decreased from its original number, in meiosis II, a process similar to mitosis, the cause of that reduction is what happens in meiosis I, i.e., the separation of the homologous chromosomes.
Photosynthesis
1. What is the primary source
of energy for living beings on
earth?
The sun, center of our planetary system and star of the milky way galaxy (our galaxy), is the source of the energy that is processed and consumed by living beings. Intense nuclear reactions in the sun liberate light and other energetic radiations into the surrounding space. Some of this energy reaches our planet.
2. How is light from the sun
transformed into chemical
energy to be used by the
living beings on earth?
Light from the sun is transformed into chemical energy contained in organic material by the photosynthesis process. In photosynthesis light, water and carbon dioxide react and highly energetic glucose molecules and molecular oxygen are made.
3. What is the chemical
equation of photosynthesis?
The chemical equation of photosynthesis is the following:
6 CO2 + 6 H2O + light --> C6H12O6 + 6
O2
4. Which are the living beings
that carry out photosynthesis?
Which is the cell organelle
responsible for the absorption
of light for the photosynthesis
process in plants and algae?
There are many beings (including all animals) that do not carry out
photosynthesis. There are also
autotrophic beings that do not perform photosynthesis but they perform
chemosynthesis. Plants, algae and cyanobacteria are photosynthetic beings.
In plants and algae, light is absorbed by chlorophyll, a molecule present in
cytoplasmic organelles called chloroplasts.
5. Are there chloroplasts in
cyanobacteria?
In cyanobacteria there are no
chloroplasts and the chlorophyll layers are dispersed in cytosol.
6. Which chemical element is
central in the chlorophyll
molecule?
The chemical element that is central in the chlorophyll molecule is magnesium. One atom of magnesium is present in the center of an amalgam of eight nitrogen-containing carbon rings.
7. How do chloroplasts
multiply?
Like mitochondria chloroplasts have their own DNA, RNA and ribosomes and they self-replicate through binary
division.
8. How can the hypothesis
that asserts that chloroplasts
as well as mitochondria were
primitive prokaryotes that
associated in mutualism with
primitive anaerobic eukaryotic
cells be corroborated?
The described hypothesis is known as the endosymbiotic hypothesis about the evolutionary origin of mitochondria and choloroplasts.
Mutualism is explained as: mitochondria and chloroplasts can offer energy and nutrients to the cell in exchange for protection. The hypothesis is
strengthened since those organelles have their own DNA, RNA and protein synthesis machinery and they divide themselves through binary division like bacteria do.