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CHAPTER 13
MEIOSIS AND SEXUAL LIFE CYCLES
• __________ is the study of heredity (transmission of
traits from one generation) and variation.
• Genes
• are segments of _____.
• program cells to synthesize specific enzymes and other
proteins that produce an organism’s inherited traits
• Consist of four __________________
• program specific traits that emerge as we develop from
fertilized eggs into adults.
• The transmission of hereditary traits has its
molecular basis in the precise __________ of DNA
• Most DNA in a eukaryotic cell is subdivided into
_________________ in the nucleus.
• Some DNA exists in ___________ and ____________.
• Every living species has a characteristic number of
chromosomes.
• Humans have 46 in almost all of their cells.
• Chromosomes consist of a single ____ molecule in
association with various ________.
• Each chromosome has hundreds or thousands of
genes, each at a specific location, it’s _____.
• ________________- occurs mostly in single-cell eukaryotes, by
mitotic cell division to produce two identical daughter cells
•_________________- results in
greater variation among offspring
than does _________reproduction.
•Offspring of sexual reproduction
vary _____________ from their
siblings and from both parents.
Fig. 13.2
1. Fertilization and ___________
• Humans contain 46 chromosomes in each _____________ (all
cells except germ cells) and 23 in the ________ (sperm or ovum).
Know these terms!
•We have ____________
________________ pairs
that carry genes that control
the same inherited characters.
•An exception - the _____
___________ (X and Y).
•Females are XX and
males are XY
_____________ - an ordered displays of an
individual’s chromosomes - used to detect large
abnormalities
Fig. 13.3
________ - Germ cells with one set of chromosomes (22
__________ and an X or a Y in humans).
________ - Somatic cells with two sets of
chromosomes (46 total in humans)
_______ - The fertilized egg
with two haploid sets of
chromosomes- one from mom,
one from dad
Gametes develop by
_________ instead of mitosis
in which the chromosome
number is halved.
Fig. 13.4z
2. Meiosis reduces chromosome number from
diploid to haploid
In meiosis, there are two consecutive cell divisions,
__________ and ___________ which results in four
haploid daughter cells.
How??- By copying chromosomes
once, but dividing twice.
Meiosis I – separates
____________________
Meiosis II-separates
______________________
Fig. 13.6
Meiosis I
• 4 phases: prophase, metaphase,
anaphase, and telophase.
___________ - Previously replicated
chromosomes join together at many
locations to exchange DNA
These exchange points are called _________
Spindles form from ___________
Metaphase, anaphase
and telophase are
similar to those in
__________
End result2 ________
cells
Metaphase Anaphase
Meiosis II
• Meiosis II is very similar to mitosis
• BUT- DNA is not ___________ first.
Prophase, metaphase,
anaphase or telophase
occur
End result4 _____ cells
Fig. 13.7
Comparison of mitosis and meiosis
End result
Meiosis
Mitosis
___
____
Daughter cells
________
________
Chiasmata?
_____
____
# of divisions
Purpose
__
________
__
_________
are the physical manifestation of crossing
over, a form of genetic rearrangement.
____________
• Mitosis produces two _________ daughter cells,
but meiosis produces 4 very different cells.
Fig. 13.8
3. Sexual life cycles produce genetic
variation among offspring
• Four mechanisms contribute to genetic variation:
1. _____________________
• A fifty-fifty chance that a
particular daughter cell of
_______ will get the
maternal chromosome of a
certain homologous pair
• For humans with n = 23,
there are about 8 ________
possible combinations of
chromosomes.
Gametes
• Four mechanisms..
(Cont.)
2. ___________________-
-homologous portions of two
nonsister chromatids trade
placesin meiosis I.
• produces ____________
____________ which combine
genes inherited from each parent.
•For humans, this occurs ______
________ per chromosome pair.
Fig. 13.10
• Four mechanisms..
(Cont.)
3. _____________________
Any sperm can fuse with any egg.
• A zygote produced by mating of a woman and man has
a unique genetic identity.
• A given ovum and sperm each have 223 (8 million)
________________________
• A zygote is composed of 1 in 70 trillion (223 x 223)
possible combinations of chromosomes.
• __________________ adds even more variation to this.
• Four mechanisms..
4. _
(Cont.)
__________________
This results in adaptation, the accumulation of
favorable genetic variations.
New genetic _________________ that work best
in the new conditions will produce more
offspring and these genes will increase.