Chapter 13 Flashcards
Living organisms are distinguished by
their ability to reproduce their own kind
Genetics is the
scientific study of heredity and variation
Heredity is the
transmission of traits from one generation to the next
Variation is
demonstrated by the differences in appearance that offspring show from parents and siblings
Offspring acquire genes from parents by
inheriting chromosomes
In a literal sense, children do not
inherit particular physical traits from their parents
It is
genes that are actually inherited
Genes are the
units of heredity, and are made up of segments of DNA
Genes are generic categories, but they have specific locations
Genes are passed to the next generation via reproductive cells called
gametes (sperm and eggs)
Each gene has a specific location called a
locus, on a certain chromosome
Most DNA is packaged into
chromosomes
One set of chromosomes is inherited from
each parent
Every person has
specific details (alleles)
In asexual reproduction (mitosis),
a single individual passes genes to its offspring without the fusion of gametes
A clone is
a group of genetically identical individuals from the same parent
In sexual reproduction,
two parents give rise to offspring that have unique combinations of genes inherited from the two parents
Fertilization and Meiosis alternate in
sexual life cycles
A life cycle is the
generation-to-generation sequence of stages in the reproductive history of an organism
Human somatic cells (any cell other than a gamete) have
23 pairs of chromosomes.
have 2 of every chromosome
A karyotype is an
ordered display of the pairs of chromosomes from a cell
The two chromosomes in each pair are called
homologous chromosomes, or homologs
Chromosomes in a homologous pair are the
same length and shape and carry genes controlling the same inherited characters ((but slightly different information??))
The sex chromosome, which determine the sex of the individual, are called
X and Y
Human females have a
homologous pair of X chromosomes (XX)
Human males have
one X and one Y chromosome (XY)
The remaining 22 pairs of chromosomes are called
autosomes
not a sex chromosome
Each pair of homologous chromosomes includes
one chromosome from each parent
The 46 chromosomes in a human somatic cell are
two sets of 23: one from the mother and one from the father
A diploid cell (2n) has
two sets of chromosomes
For humans, the diploid number is
46 (2n = 46)
In a cell in which DNA synthesis has occurred,
each chromosome is replicated
Each replicated chromosome consists of
two identical sister chromatids
A gamete (sperm or egg) contains a
single set of chromosomes, and is haploid (n)
For humans, the haploid number is
23 (n = 23)
Each set of 23 consists of
22 autosomes and a single sex chromosome
In an unfertilized egg (ovum),
the sex chromosome is X
In a sperm cell,
the sex chromosome may be either X or Y
Fertilization is the
union of gametes (the sperm and the egg)
when the egg and sperm come together
The fertilized egg is called a
zygote and has one set of chromosomes from each parent
The zygote produces
somatic cells by mitosis and develops into an adult
At sexual maturity,
the ovaries and testes produce haploid gametes
Gametes are the only types of human cells produced by
meiosis, rather than mitosis
Meiosis results in
one set of chromosomes in each gamete
Fertilization and meiosis alternate in
sexual life cycles to maintain chromosome number
The alternation of meiosis and fertilization is common to
all organisms that produce sexually
The three main types of sexual life cycles differ in
the timing of meiosis and fertilization
Gametes are the only haploid cells in
animals
The gametes are produced by meiosis and undergo no further
cell division before fertilization
Gametes fuse to form a diploid zygote that
divides by mitosis to develop into a multicellular organism
Plants and some algae exhibit an
alternation of generations
This life cycle (plants) includes both a
diploid and haploid multicellular stage
The diploid organism, called the
sporophyte, makes haploid spores by meiosis
Each spore grows by mitosis into a
haploid organism called a gametophyte
A gametophyte makes
haploid gametes by mitosis
Fertilization of gametes results in a
diploid sporophyte
In most fungi and some protists,
the only diploid stage is the single-celled zygote; there is no multicellular diploid stage
In most fungi, the zygote produces haploid cells by
meiosis
Each haploid cell grows by
mitosis into a haploid multicellular organism
The haploid adult produces gametes by
mitosis
Depending on the type of life cycle,
either haploid or diploid cells can divide by mitosis
However, only diploid cells can
undergo meiosis
In all three life cycles,
the halving and doubling of chromosomes contributes to genetic variation in offspring
Meiosis reduces the
number of chromosome sets from diploid to haploid
Like mitosis, meiosis is preceded by
the replication of chromosomes
Meiosis takes place in two sets of cell divisions, called
meiosis I and meiosis II
The two cell divisions result in
four daughter cells, rather than the two daughter cells in mitosis
Each daughter cell has only
half as many chromosomes as the parent cell
Meiosis Stages
- Replicate DNA (DNA synthesis)
- meiosis I
P-prophase I
M- metaphase I
A- anaphase I
T- telophase I and cytokinesis - meiosis II
P-prophase I
M- metaphase I
A- anaphase I
T- telophase I and cytokinesis
After chromosomes duplicate, two divisions follow
- Meiosis I (reductional division: homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes
- Meiosis II (equational division): sister chromatids separate
-The result is four haploid daughter cells with unreplicated chromosomes
Meiosis I is preceded by
interphase, when the chromosomes are duplicated to form sister chromatids
The sister chromatids are
genetically identical and joined at the centromere
The single centrosome replicates, forming
two centrosomes
Division in meiosis I occurs in four phases
P-prophase I
M- metaphase I
A- anaphase I
T- telophase I and cytokinesis
((they are generally the same as in Mitosis, but there are a few differences))
Prophase I typically occupies more than
90% of the time required for meiosis.
In prophase I,
chromosomes begin to condense
In synapsis,
homologous chromosomes loosely pair up, aligned gene by gene
In crossing over,
nonsister chromatids exchange DNA segments
Each pair of chromosomes forms a
tetrad, a group of four chromatids
Each tetrad usually has one or more
chiasmata, X-shaped regions where crossing over occurred
3 things that make meiosis different/unique from mitosis
- prophase I- synapsis/crossing over
- metaphase I- homologs line up
- Anaphase I- homologs separate
In metaphase I,
tetrads line up at the metaphase plate, with one chromosome facing each pole
Microtubules from one pole are attached to the
kinetochore of one chromosome of each tetrad
Microtubules from the other pole are attached to
the kinetochore of the other chromosome
In anaphase I,
pairs of homologous chromosomes separate
One chromosome moves toward
each pole, guided by the spindle apparatus
Sister chromatids remain attached at the
centromere and move as one unit toward the pole
In the beginning of telophase I,
each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids
Cytokinesis usually occurs
simultaneously, forming two haploid daughter cells
- In animal cells, a cleavage furrow forms
- In plant cells, a cell plate forms
No chromosome replication occurs between the
end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated
Division in meiosis II also occurs in four phases
P- prophase II
M- metaphase II
A- anaphase II
T- telophase II and cytokinesis
Meiosis II is very similar to
mitosis
In prophase II,
a spindle apparatus forms
In late prophase II,
chromosomes (each still composed of two chromatids) move toward the metaphase plate
In metaphase II,
the sister chromatids are arranged at the metaphase plate
Because of the crossing over in meiosis I,
the two sister chromatids of each chromosome are no longer genetically identical
The kinetochores of sister chromatids attach to
microtubules extending from opposite poles
In anaphase II,
the sister chromatids separate
The sister chromatids of each chromosome now move as
two newly individual chromosomes toward opposite poles
In telophase II,
the chromosomes arrive at opposite poles
Then in telophase II,
nuclei form, and the chromosomes begin decondensing
Cytokinesis separates the
cytoplasm
At the end of meiosis,
there are four daughter cells, each with a haploid set of unreplicated chromosomes
Each daughter cell is genetically distinct from
the others and from the parent cell
Mitosis conserves the number of
chromosome sets, producing cells that are genetically identical to the parent cell
Meiosis reduces the number of
chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell
Three events are unique to meiosis, and all three occur in meiosis I
- synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
- At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes
- At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate
Genetic variation produced in sexual life cycles contributes to
evolution
Mutations (changes in an organism’s DNA) are the
original source of genetic diversity
Mutations create
different versions of genes called alleles
Reshuffling of alleles during sexual reproduction produces
genetic variation
The behavior of chromosomes (DNA) during meiosis and fertilization is responsible for
most of the variation that arises in each generation
Three mechanisms contribute to genetic variation
reasons why kids look different from parents and other siblings
- independent assortment of chromosomes
- crossing over
- random fertilization
Homologous pairs of chromosomes orient
randomly at metaphase I of meiosis
In independent assortment,
each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently (differently) of the other pairs
(chromosomes can line up differently)
The number of combinations possible when chromosomes assort independently into gametes is
2^n, where n is the haploid number
For humans (n = 23), there are more than
8 million (2^23) possible combinations of chromosomes
Crossing over produces
recombinant chromosomes, which combine DNA inherited from each parent
Crossing over beings very early in prophase I, as
homologous chromosomes pair up gene by gene
In crossing over,
homologous portions of two nonsister chromatids trade places
Crossing over contributes to
genetic variation by combining DNA from two parents into a single chromosome
Random fertilization adds to
genetic variation because any sperm can fuse with any ovum (unfertilized egg)
The fusion of two gametes (each with 8.4 million chromosome combinations from independent assortment) produces a
zygote with any of about 70 trillion diploid combinations
Each zygote has a
unique genetic identity
