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Chapter 7
&
8.
Notes
Part 1, How
Cells Reproduce and Observing Patterns in Inherited Traits
I. Background Vocabulary of
nucleus and its contents
Chromosome
- (see
fig. 7.1). a DNA molecule and
associated histone proteins, or if the chromosome is replicated, two identical DNA
molecules and proteins held together at a region called the centromere.
In replicated (=duplicated) chromosomes each of the two identical
DNA molecules and the associated histone proteins is called a sister
chromatid. The structure
of chromosomes is not visible when the nucleus is not dividing.
Chromosome
number
– every kind of organism has a particular number of chromosomes.
This number is found in the nuclei of all its cells (except in
special reproductive cells). Ex.
humans 46; ferns 100’s; houseflies 12; hydra 32; mosses 12
Karyotype
- a pictorial display of an organism’s chromosomes arranged by size and
shape. See fig. 7.3 for example of human karyotypes.
Haploid
and Diploid
– through karyotype preparations, cytologists discovered that a nucleus
may contain one or two sets of
chromosomes. Human nuclei have two sets of chromosomes, that is they are diploid
[“two of each type of chromosome are present”]; only human gamete
nuclei have a single set of chromosomes and are thus called haploid ["only one of each type of chromosome is
present"].
The concept of set is indeed quite important.
Reflecting this importance, not only were new words created
(haploid and diploid) but also symbols were adopted to express this all
important concept:
n = one set, where
“n” is for set and the “one” is implied; also = haploid.
2n
= two sets, where “n” is for set; also
= diploid.
A “set” of chromosomes is one of each kind of chromosome.
Fortunately, chromosomes have specific appearances based on overall
length and centromere position. Thus,
for humans, cytologist discovered only 23 uniquely shaped chromosomes,
that is 23 chromosomes in a set. Also,
it was discovered that for every uniquely shaped chromosome there were two
chromosomes exactly alike. The
two chromosomes that correspond between sets constitute a homologous
pair of chromosomes.
Homologous
Chromosomes
– two chromosomes with the same morphology (appearance) that carry the
same sequence of genes. Homologous
chromosomes are from opposite sets of chromosomes. A pair of homologous chromosomes carries gene pairs (see
below). Homologous chromosomes may also be called homologues-chromosomes
that are homologous.
Gene
- a segment of DNA in a chromosome that influences a trait.
Junk
DNA - segments of DNA that do not influence any trait but are
important variable portions of our DNA that make DNA fingerprinting
possible (more later about this).
Allele
- a particular form of a given gene
Genotype
- the combination of alleles
Gene
Pairs - the two alleles of a gene on homologous chromosomes, if the
gene pair consists of two identical alleles (e.g. AA or aa) the genotype
is homozygous, if they are different (e.g. Aa) the genotype is heterozygous.
II.
Cell cycle: see fig. 7.2. A
symbolic way to represent the events within a cell as it prepares to
divide; for a developing embryo, most cells are busy repeating the cell
cycle. But keep in mind that
for us as adults with fully differentiated cell types, not all cells
complete the cell cycle because some specialized cells will rarely divide
again (e.g. muscle and nerve cells) or never divide again (red blood
cells), especially those cells that function only after their death, e.g.
cells that line our mouth are dead, cells that make up our hair are dead,
in plants water conducting xylem cells are dead and dead cells can’t do
the cell cycle. Even during embryonic development some cells die and
are digested by cells of the immune system (see the fingers not yet fully
formed in fig. 7.15)
The cell cycle consists of two major periods:
Interphase - a name for the period in which a cell is not dividing;
the cell is otherwise functioning metabolically, doing the things a cell
needs to do to stay alive, ATP production for example. So, interphase is
not a “resting stage” per se. It
can be considered a resting stage only from the point of view that the
cell is “resting” between divisions.
During this pause or “rest” between divisions one particular
metabolic event will occur if the cell is to divide again (see below).
The second major period of the cell cycle is cell division (typically
mitosis and cytoplasmic division).
The important thing to know about the cell cycle is this:
it is regulated and under genetic control. Many cells should
never leave the G1 phase of interphase. All chromosomes during the
G1 phase consist of single chromatids. It is during this condition
that many genes are expressed so that the cell does its job keeping our
bodies alive (e.g. Krebs Cycle and ATP production along with many other
activities). Only if the cell
needs to divide will it inter into the S phase. The events during
the S phase are intangible otherwise as they can’t be seen by human eyes.
These events are labeled “S” for synthesis.
The “S” phase of interphase is when DNA
is replicated (duplicated). This
is when chromosomes gain their sister chromatids through a duplication
process to be covered later. It
is important to lose the misconception that sister chromatids “pair
up.” Instead it is better to have
a notion of an unreplicated chromosome essentially splitting itself down the middle lengthwise and
then healing to become two chromatids held together at the centromere.
The
Cell Cycle and Cancer - if the molecular breaks are not working
during G1, uncontrolled and repeated cell divisions result and a tumor
may grow. Cells in cancerous tumors (see fig. 7.15) release
abnormal proteins some of which signal forth the growth of blood vessels
that feed the tumor. As the abnormal cells within the cancer do
not adhere to one another well, they are apt to dislodge and be carried
by the blood stream to vital organs of the body where new, life
threatening, pain causing, tumors begin. Metastasis is the
name given to the spread of cancerous cells to other parts of the body.
III.
MITOSIS - a cell division process in which one cell divides and becomes
two cells. The nuclei of the
two cells are genetic clones of one another—-that is, each nucleus has
the same DNA as the other.
Repairs
& replaces dead or injured tissue (EX. Red
Blood
Cells
live only about 120 days; Check
Cells
lining your mouth have even shorter lives)
Results
in growth of
the individual in multicellular organisms
Results
in population growth in asexually
reproducing
unicellular life, plants,
and
some animals
(recall
fragmentation and regeneration of body parts)
Produces
genetic clones of mother cell
(an
advantage in stable environments when the clone is well suited to that
environment)
Can
produce many cell types, that while they be genetic clones, the cell
types differ markedly, ex. compare these genetic clones, an epithelial
check cell seen in lab and a nerve cell that reaches from your spine to
your big toe. They look nothing alike yet carry the same genes.
Is a one
division process producing only two new cells
IV.
Meiosis. Meiosis
is one of the necessary events needed to complete sexual reproduction.
As a type of nuclear division (or karyokinesis)
meiosis is a bit more complex than mitosis.
So, one might ask what’s the benefit of a more complex division
process (meiosis) over that of a simpler division process (mitosis)? Meiosis does have beyond the events of mitosis such
occurrences as synapsis (the physical pairing of homologous
chromosomes during prophase I) and
separation of homologues (the two chromosomes of a synapsed pair
separate during anaphase I and end up in separate cells by the end of
meiosis I). You might answer
that meiosis is a two rather than one division process, that four rather
than 2 daughter cells are formed, and that chromosome numbers are reduced
by one half in the newly formed daughter cells.
These are all true but such facts do not answer the question of
what’s the benefit of meiosis over that of mitosis?
Increased genetic variation through the production of new combinations of
alleles is a direct result of meiosis, specifically crossing over during
meiosis. Crossing over is the exchange of chromosome
fragments (thus alleles) between nonsister chromosomes of a synapsed pair
of homologues. New combinations of alleles will be found on the
chromosomes after crossing over.
Both mitosis and meiosis play important roles in reproduction.
Mitosis allows for growth in multicellular organisms, repair and
replacement of damaged tissues, and asexual reproduction.
To understand the benefits of meiosis it is necessary to think of
meiosis as part of sexual reproduction [meisosis and fertilization are
the two key events defining sexual reproduction].
To put the question another way, what is the benefit of sexual
reproduction (a meiosis requiring event) over that of asexual reproduction
(a mitosis requiring event)? You
need only to complete a Punnett square for the genetic cross between
identical parents, say Aa breeds with Aa (each parent is identical, no
variation, they could be clones of one another if the species is
hermaphroditic as in earthworms). The results
of this cross should yield variation among the offspring.
Thus, sexual reproduction
via meiosis and fertilization produces
genetic variation.
MEIOSIS halves the
chromosome number.
Fertilization is another event required for sexual reproduction.
Fertilization doubles the chromosome number in the offspring.
But offspring have the same chromosome number as their parents.
Therefore, the parents, through meiosis, reduced the chromosome
number by one-halve in their cells capable of fertilization.
Following fertilization the chromosome number is restored to the diploid
number (i.e., two sets of chromosomes, 2n).
Meiosis halves the parental diploid chromosome number by simply
separating the sets of chromosomes in specialized cells in gonads
(animals) or sporangia (plants;
sporangia are sacs that produce spores,
spores are like seeds in that they grow into a new individuals). The process of separating the sets of chromosomes is called meiosis. It requires a diploid cell to undergo two divisions thus
producing four daughter cells each with a single set of chromosomes (i.e.,
haploid or n).
V. Meiosis and
Gamete
Formation in Animals
[see figs. 7.13 & 7.14]
In animals meiosis occurs in gonads and produces gametes.
This leads to a life cycle in which only gametes
are haploid.
· egg
formation proceeds via meiosis producing four haploid daughter cells of unequal size, only one of
which is a functional egg, the other three cells (called polar
bodies) wither and die.
·
sperm formation proceeds via meiosis producing four haploid daughter cells of equal size, each of which grow
a flagellum and become functioning sperm cells.
GO
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