Important words and concepts
from Chapter 13, Campbell & Reece, 2002
(1/29/2005):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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Course-external links are
in brackets Click [index] to access site index Click here to
access text’s website Vocabulary
words
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(1) Chapter title: Meiosis and Sexual Life
Cycles
(a)
[meiosis and sexual life cycles
(Google Search)] [index]
(a)
Genetics is the science of the study of heredity and variation
(b)
[genetics (Google Search)] [index]
(a)
Genes are sections (linear segments) of DNA
(b)
Typically genes specify the construction of specific proteins
(c)
Together these proteins control an organism’s inherited traits
(d)
[genes (Google Search)] [index]
(a)
Genes
have specific nucleotide sequences
(b)
Copies of these sequences are passed from parents to offspring in the
process of heredity
(c)
[heredity (Google Search)] [index]
(a)
The nucleotide sequences of genes can vary
between individuals (even within individuals)
(b)
These sequence variations are also passed on to offspring
(c)
The existence of these alternative nucleotide sequences forms the root
of quite a bit of the phenotypic variation one observes among individuals of
the same species
(d)
[variation genetic (Google Search)] [index]
(a)
A chromosome, within a eukaryotic cell, is
a single double helix
(b)
Eukaryote chromosomes have certain properties
and structures including
(i)
centromeres
(ii)
telomeres at their ends
(iii)
they are linear (rather than closed circular like bacterial
chromosomes)
(iv)
they are propelled to daughter cells during anaphase, etc.
(c)
Note that your text is using the more global definition of chromosome
at this point, i.e., more than just something that is visible during mitosis
(d)
We, however, will tend in this class to employ a more-narrow definition of eukaryotic chromosome, i.e., nuclear DNA-protein complexes that are visible
through a light microscope
(e)
[chromosome (Google Search)] [cytogenetics gallery (Department of Pathology – University of Washington)][index]
(a)
A locus is the actual, physical location of a gene on a chromosome
(b)
[locus genetic, locus chromosome (Google Search)] [index]
(a)
A karyotype is an arrangement of chromosomes, in
order from largest to smallest, on a photographic plate
(b)
Karyotypes separate chromosomes into different types based on
(i)
Size
(ii)
Staining (i.e., banding) patterns
(iii)
location of the centromere (e.g.,
near one end versus near middle)
(c)
See Figure on lower right of
page 234
(d)
[karyotype (Google Search)] [karyotypes links (MicroDude)] [index]
(9) Homologous
chromosomes (homologues)
(a)
A cell that contains two or more of a given type of chromosome is said to contain two or more homologous chromosomes
(b)
Note that homologues are not necessarily identical (i.e., do not
necessarily possess exactly the same nucleotide
sequences, without variation), but do tend to be very, very,
similar
(c)
Alls cells from normal humans contain at least 22 pairs of homologous
chromosomes (cells from human females contain 23)
(d)
[homologous chromosomes,
homologue or homologues
(Google Search)] [index]
(a)
A cell that contains only one of each type of chromosome that is typical for its species is said to be
haploid
(b)
For humans, a haploid cell possesses 23 chromosomes
(c)
That is, a total of 23 distinguishable types of chromosomes
(d)
Another way of saying this is that n = 23
(e)
[haploidy, haploid (Google Search)] [index]
(a)
Most human cells are diploid
(b)
This means that the cell contains two sets of chromosomes, i.e.,
two haploid
sets
(c)
That is, a total of 46 chromosomes
(d)
2n = 46
(e)
These 46 chromosomes are, in turn, distinguishable into 23 pairs of homologues (with one exception, see below)
(f)
[diploidy, diploid (Google Search)] [index]
(a)
The majority of chromosomes in a diploid cell exist,
under all normal circumstances, as one half of a homologous pair
(b)
Such chromosomes are termed autosomes
(c)
Cells from normal humans contain 44 autosomes (male or female)
(d)
[autosome or autosomes
(Google Search)] [index]
(a)
Exceptions to the two-of-each-type-of-chromosome rule are
the sex chromosomes
(b)
In particular, in humans and other mammals the sex chromosomes are
found as a non-homologous pair in males
(c)
That is, a haploid of the chromosome termed X and a
haploid of the chromosome termed Y are found in males
(d)
Females possess a homologous pair of X chromosomes (and hence human
females possess 23 pairs of homologous chromosomes)
(e)
[sex chromosomes (Google Search)] [index]
(a)
Asexual reproduction involves
(i)
No change in ploidy going from parent to offspring
(ii)
Little genetic
variation
between parent and offspring
(iii)
Only mitotic division between parent and offspring
(among eukaryotes at least)
(b)
[asexual reproduction,
asex (not nearly as
popular as "sex" but still a term used by some) (Google Search)] [asexual reproduction biology
(includes a number of plants "doing it") (The Biology File)]
[Human Cloning Foundation (is this for
real?)] [index]
(a)
Sexual reproduction involves
(i)
Changes in ploidy over the course of going from parent to offspring
(ii)
Significant genetic variation between parent and
offspring
(iii)
Both mitotic and meiotic division
between parent and offspring
(b)
[sexual reproduction,
"sex" alone had
nearly 35 million google hits 2/2/2001, over 87 million hits as of 2/13/02, and
about 262 million hits on 2/9/04—that’s an approximate doubling per year! (Google Search)] [index]
(a)
A life cycle is a description of an organism as it passes from
conception to production of progeny (offspring)
(b)
The human life cycle is divided between a somatic stage and a
gamete
stage
(c)
See Figure 13.4, The human
life cycle
(d)
[life cycle (Google Search)] [the haploid-diploid cycle of
sexual reproduction (Access Excellence)]
[index]
(a)
The soma, or somatic cells of a human are mostly diploid (or possess
a higher ploidy)
(b)
These are the cells that constitute the majority of your body
(c)
The evolutionary goal of your soma is the make gametes and then to assure their fertilization and
subsequent development, thereby contributing genetically to the next generation
(d)
[soma cell, somatic cell, somatic cells (Google Search)] [index]
(a)
A minor portion of your somatic cells give rise to your gametes
(b)
These cells are called your germ-line cells
(c)
Note that the majority of your cells are not capable of making gametes,
only cells from your germ line are
(d)
[germ line (Google Search)] [index]
(a)
Gametes are haploid cells
(b)
They are called sperm and ova (coming from the male and the female,
respectively)
(c)
Gametes do not divide
(d)
In humans, gametes are generated by meiosis
(e)
[gamete or gametes (Google Search)] [index]
(a)
All sexual life cycles are characterized by an occurrence of meiosis
(b)
Meiosis specifically converts the diploid stage of an
organism to its haploid stage
(c)
In humans, only germ line cells undergo meiosis
(d)
Anatomically, this occurs in the sperm- or ova-generating testes or
ovaries
(e)
See Figure 13.6, Overview of
meiosis: how meiosis reduces chromosome number
(f)
(below meiosis is considered in greater detail)
(g)
[meiosis (Google Search)] [index]
(a)
Fertilization (a.k.a., syngamy) is also involved in all sexual life cycles
(b)
Fertilization specifically converts the haploid stage of an
organism to its diploid stage
(c)
That is, the haploid to diploid conversion during a life cycle in most
organisms is a result of fertilization
(d)
[fertilization, syngamy (Google Search)] [insemination and fertilization
(The San Francisco Center for Reproductive Medicine)]
[index]
(22) Variety of
sexual life cycles
(a)
The human life cycle is only one of a variety of sexual cycles
(b)
Different sexual life cycles vary in terms of whether haploid
or diploid
(or both) cells undergo mitosis between fertilization and meiosis and fertilization
(c)
In humans (and most though not all other animals) only the diploid
stage is capable of undergoing mitosis; the haploid cells do not divide
(d)
See Figure 13.5a, Three
sexual life cycles in the timing of meiosis and fertilization (syngamy)
(e)
[variety of sexual life cycles
(Google Search)] [index]
(a)
Various fungi, protozoa, and
algae do not undergo mitosis during their diploid stage
(b)
Instead they undergo mitosis only during their haploid stage
(c)
See Figure 13.5b, Three
sexual life cycles in the timing of meiosis and fertilization (syngamy)
(d)
Note that this does not mean that gametes are
replicating
(e)
Instead it means that gametes (i.e., cells capable of undergoing fertilization)
are generated by mitosis rather than by meiosis (that is,
if the immediate cellular parent of a gamete is itself haploid then the gamete
must have been the product of mitosis)
(f)
Depending on the organism, the haploid stage may or may not remain
unicellular, i.e., there exist multicellular haploid organisms
(24) Alternation of generations
(a)
Plants and various species of algae undergo both haploid and diploid
mitosis
(b)
This is termed alternation of generations since both the haploid and
the diploid organisms are considered separate generations
(c)
See Figure 13.5c, Three
sexual life cycles in the timing of meiosis and fertilization (syngamy)
(d)
[alternation of generations
(Google Search)] [index]
(a)
The multicellular diploid organism given alternation of
genertations is termed the sporophyte
(b)
Sporophytes give rise to haploid spores which go on to found the haploid
organism
(c)
[sporophyte (Google Search)] [index]
(a)
The multicellular haploid organism given alternation of generations is
termed the gametophyte
(b)
The gametophytes generate gametes by mitosis
(c)
Gametes don’t divide but instead undergo fertilization to
produce the next sporophyte generation
(d)
[gametophyte (Google Search)] [index]
(a)
See Figure 13.7, The stages
of meiotic cell division
(b)
See Figure 13.8, A
comparison of mitosis and meiosis
(c)
Note that much of meiosis resembles mitosis
(d)
Exceptions include:
(i)
Meiosis involves two stages of division, meiosis I and meiosis II
(ii)
Prophase I of meiosis
involves chiasma formation
(iii)
Metaphase I involves the lining up of tetrads rather than simply sister chromatid pairs (i.e., the latter as
seen in mitosis)
(iv)
Anaphase I involves the separation of homologues,
not the separation of sister chromatid pairs (i.e., the latter as
seen in mitosis)
(v)
Meiosis II is essentially the mitotic division of a haploid
cell
(e)
Note that I have an expectation that you will basically learn (i.e.,
memorize and understand) Figure 13.6 of your text
(f)
Note that the diploid to haploid transition in sexual life cycles occurs via meiosis
(g)
[meiosis (Google Search)] [meiosis (Biology at Clermont College)] [index]
(a)
Interphase I is essentially similar to that which occurs prior to mitosis
(b)
[interphase, interphase I (Google Search)] [index]
(a)
Meiosis I includes the following phases: prophase I, metaphase
I, anaphase I, and telophase I
(b)
[meiosis, meiosis I (Google Search)] [index]
(a)
Unlike mitosis prophase (and prometaphase), prophase I involves the synapsis
of homologous chromosomes to form tetrads
(b)
Chiasmata form between the homologous chromosomes
(c)
Otherwise Prophase I resembles prophase and prometaphase
of mitosis
(d)
[prophase, prophase I (Google Search)] [index]
(a)
Synapsis is the pairing up of homologues that
occurs during prophase I of meiosis
(b)
[synapsis (Google Search)] [index]
(a)
Upon synapsis two paired-up chromosomes form a tetrad
(b)
A tetrad is thus a structure consisting of two, joined (synapsed) homologous chromosomes (i.e., two homologous sister-chromatid pairs), i.e., four double helicies
(c)
[tetrad meiosis (Google Search)] [index]
(33) Chiasmata (singular, chiasma)
(a)
Within a tetrad the arms of homologous chromosomes pair up as chiasmata
(b)
See Figure 13.10, The
results of crossing over during meiosis
(c)
[chiasmata (Google Search)] [tetrads and chiasmata
(Basic Genetics)] [index]
(a)
In metaphase I tetrads line up in the metaphase plate rather than individual sister chromatid pairs lining up (i.e., the
latter as seen in mitosis)
(b)
[metaphase, metaphase I (Google Search)] [index]
(a)
In anaphase I tetrads are broken up into two sister chromatid pairs, each of which is pulled
toward opposite centrosomes
(b)
[anaphase, anaphase I (Google Search)] [index]
(a)
The new cell upon completion of telophase I now has an already
replicated, haploid set of chromosomes
(b)
[telophase, telophase I (Google Search)] [index]
(a)
Two cells are present following a round of cytokinesis
(b)
[cytokinesis (Google Search)] [index]
(a)
Interphase II may or may not be present
(b)
Note that synthesis of DNA does not occur during interphase II
(c)
Sythesis occurs only prior to meiosis I, not
between meiosis I and meiosis II
(d)
[interphase II (Google Search)] [index]
(a)
Meiosis II is the essentially mitotic division of
the haploid
set of chromosomes present during telophase I
(b)
Phases include: prophase II, metaphase II, anaphase II, telophase II
(c)
Don’t worry about the lack of a prometaphase II; that is contained
within prophase II (just as mitosis used to be
taught)
(d)
[meiosis II (Google Search)] [index]
(a)
Cytoplasmic division at the end of meiosis II divides cells into a total of four haploid
cells
(b)
[cytokinesis (Google Search)] [index]
(a)
Sexual reproduction leads to genetic variation
in offspring by three mechanisms
(ii)
Crossing over
(iii)
Random fertilization
(b)
It is important to keep in mind, however, that the ultimate root of all
variation
is mutation (which will be considered in a subsequent chapter)
(c)
[genetic variation (Google Search)] [index]
(a)
Each of your cells possesses two sets of chromosomes
(b)
One set came from your father
(c)
The other set came from your mother
(d)
The products of meiosis contain, with high probability, some
mixture of the chromosomes from your
father and your mother, rather than a
complete set from one and an absence of the other
(e)
This is true even without a consideration of recombination/chiasmata/crossing
over.
(f)
See figure 13.9, The results
of alternative arrangements of two homologous chromosome pairs on the metaphase
plate in meiosis I
(g)
This redistribution of your parent’s chromosomes occurs
during anaphase I
(h)
It occurs because each homologue that makes up a tetrad progresses to a separate, randomly chosen pole of the
cell during anaphase I
(i)
Thus, for any given pair of chromosomes, one pair of sister chromatids, coming from your mother,
will go to one daughter cell and the other pair of sister chromatids, coming
from your father, will go to the other daughter cell
(j)
Multiplied over all of the types of chromosomes in your body, the end
result is a random mixing of the chromosomes coming from each of your parents
during anaphase I
(k)
Considering 23 pairs of chromosomes and no recombination, the
likelihood that any one product of meiosis
I
will contain chromosomes only coming from your father is [(½ * ½ * ½ * ½ * ½ *
½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½) = (½)23
= 0.00000012] and the same value (probability) for
those coming only from your mother.
(l)
[independent assortment
(Google Search)] [index]
(43)
Crossing over (recombination)
(a)
Meiosis I doesn’t furnish offspring with genetic variation just by independent assortment
(b)
This is because, in addition to the separation from each other of chromosomes
coming from each parent, meiosis I additionally mixes together individual
chromosomes
(c)
In this way, each chromosome is no longer descended only from one of
each parent but instead is descended from a combination of both parents
(d)
This mixing up occurs randomly
(e)
Consequently, different cells independently assort a randomly-varying
combination of chromosomes, thus greatly increasing the possible genetic
variation seen in offspring
(f)
See Figure 13.10, The
results of crossing over during meiosis
(g)
Note that all of this crossing over occurs during prophase I
(h)
During prophase I, homologous chromosomes
synapse
to form tetrads
(i)
Within tetrads, chromosomes form chiasmata
(j)
Chiasmata are a physical manifestation of the process of crossing over
(k)
[crossing over meiosis,
recombination (Google Search)] [index]
(a)
As a consequence of independent assortment
and crossing over, each gamete is randomly unique (i.e.,
genetically distinct from most or, more likely, all other gametes)
(b)
Fertilization then occurs as a random coming together of
genetically unique gametes
(c)
Note that some fertilizations are more unique than others: A process
that can lead to a reduction in the random variation present between randomly
fertilizing gametes is inbreeding (but we
will consider that at a later date)
(d)
[random fertilization
(Google Search)] [index]
(45)
(If there is time, I will introduce probability theory which otherwise is found in chapter 14)
(46) Vocabulary [index]
(a)
Alternation of generations
(b)
Anaphase I
(d)
Autosome
(e)
Chiasma
(f)
Chiasmata
(g)
Chromosome
(h)
Crossing over
(i)
Cytokinesis (1)
(j)
Cytokinesis (2)
(k)
Diploidy
(l)
Fertilization
(m)
Gametes
(n)
Gametophyte
(o)
Genes
(q)
Genetics
(r)
Germ line
(s)
Haploidy
(v)
Homologues
(x)
Interphase I
(y)
Interphase II
(z)
Karyotype
(aa)
Life cycle
(bb)
Loci
(dd)
Meiosis
(ee)
Meiosis I
(ff)
Meiosis II
(gg)
Meiosis in detail
(hh)
Metaphase I
(ii)
Prophase I
(jj)
Random
fertilization
(kk)
Recombination
(ll)
Replicating
haploid
(mm)
Sex chromosomes
(nn)
Sexual
reproduction
(pp)
Somatic cells
(qq)
Sporophyte
(rr)
Synapsis
(ss)
Syngamy
(tt)
Telophase I
(uu)
Tetrad
(vv)
Variation