Transcript Meiosis - Grant County Schools

Organisms have tens of
thousands of genes
- Genes determine traits
- Genes are lined up on
chromosomes
- 1 chromosome can contain
a thousand or more genes
 Each species of organisms contains a different number
of chromosomes
 The chromosome numbers of a species is not related
to the complexity of the organism
The Chromosomes in
most plants and
animals occur in pairs
•
•
1 chromosome comes
from the male parent
1 chromosome comes
from the female parent
Diploid – A cell with two
of each kind of
chromosome
Haploid – A cell with one
of each kind of
chromosome (gametes)
(1 from each parent)
We say the cell is a diploid
cell or 2n
*Supports Mendel’s conclusion
that organisms have two
factors (alleles) for each trait
We say the cell is a haploid
or n
*Supports Mendel’s conclusion
that parents give 1 allele for each
trait to their offspring
Homologous Chromosomes – Paired
chromosomes
Each pair of homologous chromosomes has genes
for the same traits
Ex. Pod shape
On homologous chromosomes, the genes are
arranged in the same order
Because there are different possible alleles for the
same gene, the two chromosomes in the
homologous pairs are not always identical to each
other.
Meiosis – cell division .
-produces haploid gametes
 Meiosis occurs in the body cells that produce gametes
 (Male and Female reproductive organs)
 The process of meiosis produces:
 male gametes (sperm) - n, haploid
 female gametes (eggs) – n, haploid
 When the sperm fertilizes the egg, the resulting
cell is a Zygote – 2n diploid
 The zygote then can develop by mitosis into a
multicellular organism
 This process is called Sexual Reproduction
 Two parents produce an offspring that is not identical to
either parent
 What are the benefits to sexual reproduction?
Gamete Production
Meiosis Square Dance
•Write down the phases and
what is taking place as you
watch the video.
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Meiosis
 Similar to mitosis
 However, several differences:
 Meiosis occurs in 2 phases (2
cell divisions: Meiosis I &
Meiosis II
 Results in 4 cells
 Vocabulary:
 Diploid (2N) –
Normal amount of genetic material
 Haploid (N) 1/2 the genetic material.
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Meiosis
 Meiosis forms haploid cells.
 Example In Humans: Ova (egg) and
sperm.
 Ova produced in ovaries (females)
 Process is called oogenesis
 Sperm produced in testes (males)
 Process is called spermatogenesis
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Spermatogenesis & Oogenesis
Sperm
formation
Egg
formation
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Overview of Meiosis
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Meiosis I
 Prior to division (S phase), amount of
DNA doubles
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Metaphase I
 During Metaphase I
homologous
chromosomes lineup along the
EQUATOR of the
cell
 Areas of
homologous
chromosomes
connect at areas
called CHIASMATA
 Genes are
exchanged at
these connections
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Crossing Over
 Segments of
homologous
chromosomes break
and reattach at
similar locations.
 Results in new
genetic
combinations of
offspring.
 This is the main
advantage of
sexual reproduction
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Chiasmata
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Anaphase I
 During Anaphase I, each
HOMOLOGOUS
CHROMOSOME is pulled to
opposite sides of the cell.
 Unlike mitosis, the
CENTROMERES DO NOT
BREAK.
 Nuclei MAY OR MAY NOT
reform following division.
 CYTOKENESIS may or may
not occur.
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Meiosis II
 DNA DOES NOT double
 Chromosomes randomly line-up
along metaphase plate like
regular mitosis.
 During Anaphase II,
CENTROMERES BREAK and
each SISTER CHROMATID is
pulled to opposite sides of the
cell.
 Nuclei reform and cytokinesis
usually occurs (although it is
often unequal).
Sister chromatids
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Overview of Meiosis
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