Concept 13.2: Fertilization and meiosis alternate in sexual life cycles Flashcards
is the generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.
life cycle
In humans, each somatic cell has
46 chromosomes
During mitosis, the chromosomes become ___________ enough to be visible under a light microscope.
condensed
At this point, they can be distinguished from one another by their
size, the position of their centromeres, and the pattern of colored bands produced by certain chromatin-binding stains.
Careful examination of a micrograph of the 46 human chromosomes from a single cell in mitosis reveals that there are
two chromosomes of each of 23 types.
This becomes clear when images of the chromosomes are arranged in pairs, starting with the
longest chromosomes.
The resulting ordered display is called a
karyotype
the two chromosomes of a pair have the same length, centromere position, and staining pattern: These are called
homologous chromosomes (or homologs)
Both chromosomes of each pair carry
genes controlling the same inherited characters.
if a gene for eye color is situated at a particular locus on a certain chromosome, then its homologous chromosome (its homolog) will also have a
version of the eye-color gene at the equivalent locus.
figure 13.3 Research method
.
The two chromosomes referred to as X and Y are an important exception to the general pattern of
homologous chromosomes in human somatic cells.
Typically, human females have a homologous pair of X chromosomes (XX), while males have
one X and one Y chromosome
Only small parts of the X and Y are
homologous.
Most of the genes carried on the X chromosome do not have counterparts on the tiny Y, and the Y chromosome has genes lacking on the
X.
Due to their role in sex determination, the X and Y chromosomes are called
sex chromosomes
The other chromosomes are called
autosomes.
The occurrence of pairs of homologous chromosomes in each human somatic cell is a consequence of our
sexual origins
We inherit one chromosome of a pair from each
parent
thus, the 46 chromosomes in our somatic cells are actually
two sets of 23 chromosomes—a maternal set (from our mother) and a paternal set (from our father).
The number of chromosomes in a single set is represented by
n
Any cell with two chromosome sets is called a ______________ and has a diploid number of chromosomes, abbreviated 2n.
diploid cell
For humans, the diploid number is 46(2n=46) , the number of
chromosomes in our somatic cells.
In a cell in which DNA synthesis has occurred, all the chromosomes are duplicated, and therefore each consists of
two identical sister chromatids, associated closely at the centromere and along the arms.
figure 13.4, bioflix chromosomes
Unlike somatic cells, gametes contain a single set of chromosomes. Such cells are called _________________, and each has a haploid number of chromosomes (n).
haploid cells
For humans, the haploid number is
23(n=23)
The set of 23 consists of the 22 autosomes plus a
single sex chromosome.
An unfertilized egg contains an X chromosome; a sperm contains either an
X or a Y chromosome.
Each sexually reproducing species has a characteristic
diploid and haploid number
the fruit fly Drosophila melanogaster has a diploid number
(2n) of 8 and a haploid number (n) of 4
for dogs, 2n is 78 and n is
39
The chromosome number generally does not correlate with the
size or complexity of a species’ genome
it simply reflects how many linear pieces of
DNA make up the genome,
The human life cycle begins when a haploid sperm from the father fuses with a
haploid egg from the mother
This union of gametes, culminating in fusion of their nuclei, is called
fertilization
The resulting fertilized egg, or _________, is diploid because it contains two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines
zygote
As a human develops into a sexually mature adult, mitosis of the zygote and its descendant cells generates all the
somatic cells of the body.
Both chromosome sets in the zygote and all the genes they carry are passed with precision to the
somatic cells.
figure 13.5, animation the human life cycle
The only cells of the human body not produced by mitosis are the gametes, which develop from specialized cells called
germ cells in the gonads—ovaries in females and testes in males
gamete formation involves a type of cell division called
meiosis
This type of cell division reduces the number of sets of chromosomes from two to one in the gametes, counterbalancing the doubling that occurs at
fertilization.
As a result of meiosis, each human sperm and egg is
haploid (n=23) .
Although the alternation of meiosis and fertilization is common to all organisms that reproduce sexually, the timing of these two events in the life cycle varies, depending on the
species.
These variations can be grouped into three main types of
life cycles.
n the type that occurs in humans and most other animals, gametes are the only
haploid cells
occurs in germ cells during the production of gametes, which undergo no further cell division prior to fertilization.
Meiosis
After fertilization, the diploid zygote divides by mitosis, producing a
multicellular organism that is diploid.
figure 13.6
Plants and some species of algae exhibit a second type of life cycle called
alternation of generations
This type includes both
diploid and haploid stages that are multicellular
the multicellular diploid stage is called the
sporophyte.
Meiosis in the sporophyte produces haploid cells called
spores
Unlike a gamete, a haploid spore doesn’t fuse with another cell but divides mitotically, generating a multicellular haploid stage called the
gametophyte.
Cells of the gametophyte give rise to gametes by
mitosis.
Fusion of two haploid gametes at fertilization results in a
diploid zygote, which develops into the next sporophyte generation.
Therefore, in this type of life cycle, the sporophyte generation produces a gametophyte as its offspring, and the gametophyte generation produces the next
sporophyte generation
The term ______________________________ fits well as a name for this type of life cycle.
alternation of generations
A third type of life cycle occurs in most
fungi and some protists, including some algae
After gametes fuse and form a diploid zygote, meiosis occurs without a
multicellular diploid offspring developing.
Meiosis produces not gametes but haploid cells that then divide by mitosis and give rise to either
unicellular descendants or a haploid multicellular adult organism.
Subsequently, the haploid organism carries out further mitoses, producing the cells that develop into
gametes.
The only diploid stage found in these species is the
single-celled zygote.
Note that either haploid or diploid cells can divide by mitosis, depending on the type of life cycle. Only diploid cells, however, can undergo meiosis because
haploid cells have only a single set of chromosomes that cannot be further reduced
