Meiosis and Sexual Life Cycle Flashcards
1
Q
Why is meiosis necessary
A
To reduce the chromosome number
2
Q
Why is it necessary to reduce the chromosome number?
A
To make gametes (eggs and sperm)
3
Q
Why is it necessary to make gametes?
A
To complete the sexual life cycle
4
Q
a organism passes all of its genes to its offspring
A
asexual reproduction
5
Q
, a small freshwater animal, reproduces by budding; offspring are therefore clones, all genetically identical; many simple animals reproduce asexually
A
Hydra
6
Q
is when an organism produces sex cells called gametes
A
Sexual reproduction/meiosis
7
Q
The gametes combine at fertilization to form a
A
zygote
8
Q
The female and male gametes may be from __ individuals or, in plants, from the __ individual
A
different, same
9
Q
Zygote eventually turns into an adult by
A
mitotic cell divisions
10
Q
The cells of the human body (the somatic cells) have __ sets of chromosomes
A
2
11
Q
One set came from the mother and was in the egg
A
maternal chromosomes
12
Q
other set came from the father and was in the sperm;
A
Paternal chromosomes
13
Q
A set has how many chromosomes
A
23
14
Q
double set?
A
46
15
Q
Human cells contain a maternal chromo-some 1 and a paternal chromosome 1
A
homologous chromosomes (or just homologs)
16
Q
are the same length, have the same centromere position and carry the same genes in the same order
A
chromosomes in a homologous pair
17
Q
humans, there are __ homologous pairs of chromosomes
A
23
18
Q
X and Y in most animals in and some plants
A
sex chromosomes
19
Q
Females have a homologous pair of X chromosomes (XX);
A
homogametic sex
20
Q
Males have one X and one Y (XY);
A
heterogametic sex
21
Q
The other 22 pairs of chromosomes are called
A
autosomes
22
Q
Organisms with two sets of chromosomes are said to be
A
diploid or 2n
23
Q
diploid number in humans?
A
46, 2n=46
24
Q
make gametes with a single set of chromosomes
A
diploid organisms
25
Gametes are thus
haploid
26
In humans, the haploid number is
23 (n = 23)
27
Disadvantages of the sexual life cycle
more complicated, requiring meiosis
meiosis is error-prone
takes more energy
may entail more risk
often requires two individuals
may spread disease
causes a loss of genetic representation
involves a “two-fold cost of sex”
28
only half of each parent’s genes are passed to the offspring; “loss of genetic representation”
In sexual reproduction
29
all the genes are passed to the offspring
asexual reproduction
30
asexually reproducing organisms can potentially produce offspring much faster than sexually reproducing organisms
two-fold cost of sex
31
Do asexuals or sexuals produce faster?
asexuals
32
is necessary to make haploid gametes
meiosis
33
Cutting the chromosome in half by ____ maintains the diploid (2n) chromosome number
meiosis
34
involves copying the chromosomes once (in the S phase of interphase) followed by two divisions of the chromosomes:
A meiotic cell cycle
35
the chromosome number is reduced from 2n to n
meiosis I
36
in the chromosome number is unchanged
meiosis II
37
One diploid (2n) cell will thus divide twice to produce
four cells, each haploid (n)
38
Before the S phase, each chromosome consists of one
one chromatid
39
After the S phase, each chromosome consists of two
identical sister chromatids
40
two sister chromatids are held together at the
centromere
41
Chromatids from homologous chromosomes are called
nonsister chromatids
42
are not identical; their DNA sequences will differ
Nonsister chromatids
43
the chromosomes are copied once and divided twice
meiotic division
44
the chromosomes are copied once and divided once
mitotic division
45
preceded by an interphase
meiosis
46
the chromosomes are copied
During S phase of interphase
47
each chromo-some consists of two identical sister chromatids
At the end of the S phase
48
the homologous chromosomes pair up
they are then pulled apart
this cuts the chromosome number in half (2n-->n)
Meiosis I
49
the two sister chromatids making up each chromosome are pulled apart
this does not further reduce the chromosome number (still n)
Meiosis II
50
Preceded by interphase
Homologous chromosomes pair up
Maternal #1 pairs up with paternal #1, maternal #2 pairs up with paternal #2, etc
Prophase I
51
The phase that Chromatids from homologous chromosomes overlap, break and rejoin
Most complex
This is called crossing over. Prophase I
52
Each pair of homologous chromosomes aligns on the equatorial plane of the cell
The homologous chromosomes are now ready to be pulled apart…
Metaphase I
53
The homologous chromosomes are pulled apart
The maternal chromosome goes to one pole and the paternal chromosome to the other pole
This cuts the chromosome number in half
The original cell had two sets of chromosome (2n)
All subsequent cells will have one set (n)
Anaphase I
54
The chromosomes arrive at the opposite poles
Nuclear membranes form
The cell now divides into two cells, each with one set of chromosomes
Telophase I and Cytokinesis
55
After telophase I, there may be a short resting phase
interkinesis
56
The chromosomes may condense further
Prophase II
57
The chromosomes align on the equatorial plane
Metaphase II
58
The two sister chromatids are pulled apart and move to opposite poles
Anaphase II
59
The chromosomes arrive at the opposite poles and nuclear membranes form
Telophase II
60
The cell membrane pinches the cell into two new daughter cells
Cytokinesis
61
Occurs in only in reproductive tissues (ovaries and testes
Meiotic cell cycle
62
independent assortment of homologous chromosomes during meiosis I
The sexual life cycle generates and maintains genetic diversity
63
random fertilization between gametes
The sexual life cycle generates and maintains genetic diversity
64
crossing over between homologous chromosomes during prophase I of meiosis
The sexual life cycle generates and maintains genetic diversity
65
maternal vs. paternal chromosomes occurs independently for all the pairs of homologous chromosomes
chromosome assortment
66
At the end of meiosis there are ___ combinations of maternal and paternal chromosomes in the gametes, each occur-ring with equal probability:
8 possible
67
The number of combinations of maternal and paternal chromosomes in the gametes is 2n, where n is the haploid number of chromosomes
For n = 3, 2(3) = 8
68
how many possible chromosomal combinations in every fertilizations
70 trillion
69
crossing over occurs between what?
sister chromatids of homologous chromosomes
70
during crossing over, how many of four gametes will be recombined
2
71
Purpose of crossing over?
increases total number of possible genetic combinations
72
The point of crossing over is called
chiasma (plural chiasmata)
73
Chiasmata are visible under the microscope during the later stages of what phase
Prophase I
74
crossing over between homologous chromosomes during meiosis (prophase I)
causes genetic uniqueness
75
independent assortment of homologous chromosomes during meiosis (meta-phase & anaphase I)
causes genetic uniqueness
76
the fusion of gametes made by different organisms (fertilization)
causes genetic uniqueness
77
each parent donates his or her own set of genetic material
Alleles come from two parents
78
The homologs and sister chromatids distributed to each daughter cell during meiosis are a random mix of maternal and paternal genetic material
assortment of homologues
79
So why don’t sexually reproducing organisms give up sex and reproduce asexually?
Females could make “eggs” without reducing the chromosome number by meiosis.
80
Fertilization would become unnecessary.
Females would give birth to their genetic clones, all daughters
parthenogenesis
81
common in some insects, including aphids and some bees, but is rare among vertebrates
parthenogenesis
82
asexual animal
15 species are all-female
a courtship ritual is still required for ovulation
American whiptail lizard
83
Suggested advantages of sex
It generates and maintains genetic diversity.
It eliminates harmful mutant alleles.
It increases the resistance to harmful parasites.
84
Meiosis shuffles the chromosomes
Crossing over further recombines the genes
Fertilization brings together gametes from unrelated individuals
The offspring are genetically unique: different from each other and from both parents
The new genetic combinations created each generation may allow rapid adaptation to changing environments
Generation of genetic diversity
85
Meiosis with crossing over can bring the most beneficial alleles from the maternal and paternal chromosomes together onto one chromosome
Generation of genetic diversity
86
Mutations occur during the lifetime of an organism, creating harmful alleles in cells throughout the body
Elimination of harmful mutant alleles
87
By asexual reproduction, these “somatic mutations” might be passed to the off-spring
Elimination of harmful mutant alleles
88
In sexual reproduction, the genes are passed to the next generation through a single cell, the gamete
Elimination of harmful mutant alleles:
89
The gamete would not have any of the somatic mutations
The offspring thus start with a “clean slate”
Elimination of harmful mutant alleles
90
the number of mutations “ratchets up” irreversibly over time
Muller's Ratchet
91
Harmful parasites may infect cells throughout the body
By asexual reproduction, these parasites could be passed to the offspring
In sexual reproduction, the genes are passed to the next generation through a single cell, the gamete
Increased resistance to parasites
92
The gamete would be free of the para-sites
The offspring thus start with a “clean slate”
The shuffling of alleles by meiosis would also create combinations that give greater resistance to parasites
Increased resistance to parasites
