s7l3. students will recognize how biological traits are passed on to successive generations....
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S7L3. Students will recognize how biological traits are passed on to successive generations.
A.Explain the role of genes and chromosomes in the process of inheriting a specific trait.B. Compare and contrast that organisms reproduce asexually and sexually (bacteria, protists, fungi, plants, and animals). C. Recognize that selective breeding can produce plants or animals with desired traits.
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MEIOSIS
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Organisms that reproduce Sexually are made up of two different types of cells.
1. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc.
2. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova are gametes.
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Gametes• The Male Gamete is the Sperm and is
produced in the male gonad the Testes.
• The Female Gamete is the Ovum (ova = pl.) and is produced in the female gonad the
Ovaries.
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During Ovulation the ovum is released from the ovary and transported to an area where fertilization, the joining of the
sperm and ovum, can occur…… fertilization, in Humans, occurs in the Fallopian tube. Fertilization results in the formation of the
Zygote. (fertilized egg)
Sperm + Ovum (egg) Zygotefertilization
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Fertilization
• The fusion of a sperm and egg to form a zygote.• A zygote is a fertilized egg
n=23egg
sperm n=23
2n=46zygote
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Chromosomes• If an organism has the Diploid number (2n) it has
two matching homologues per set. One of the homologues comes from the mother (and has the
mother’s DNA).… the other homologue comes from the father (and has the father’s DNA).
• Most organisms are diploid. Humans have 23 sets of chromosomes… therefore humans have 46 total chromosomes….. The diploid number for
humans is 46 (46 chromosomes per cell).
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Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are similar in shape and size.
• Homologous pairs (tetrads) carry genes controlling the same inherited traits.
• Each locus (position of a gene) is in the same position on homologues.
• Humans have 23 pairs of homologous chromosomes.
22 pairs of autosomes
1 pair of sex chromosomes
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Homologous Chromosomes(because a homologous pair consists of 4 chromatids it is called a “Tetrad”)
Paternal Maternal
eye color locus
eye color locus
hair color locus
hair color locus
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Karyotype(picture of an individual’s chromosomes)
One of the ways to analyze the
amniocentesis is to make a Karyotype
What genetic disorder does this karyotype show?
Trisomy 21….Down’s Syndrome
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Humans have 23 Sets of Homologous ChromosomesEach Homologous set is made up of 2 Homologues.
Homologue
Homologue
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Autosomes(The Autosomes code for most of the offspring’s traits)
In Humans the “Autosomes” are sets 1 - 22
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Sex ChromosomesThe Sex Chromosomes code for the sex of the offspring.
** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y”
chromosome it will be a male.
XX chromosome - female XY chromosome - male
In Humans the “Sex Chromosomes” are
the 23rd set
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Boy or Girl? The Y Chromosome “Decides”
X chromosome
Y chromosome
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Meiosis is the process by which ”gametes” (sex cells) , with half
the number of chromosomes, are produced.
During Meiosis diploid cells are reduced to haploid cells
Diploid (2n) Haploid (n)
If Meiosis did not occur the chromosome number in each new generation would
double…. The offspring would die.
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MeiosisMeiosis is Two cell divisions
(called meiosis I and meiosis II)
with only one duplication of chromosomes.
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Meiosis in males is called spermatogenesis and produces sperm.
Meiosis in females is called oogenesis and produces ova.
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Spermatogenesis
2n=46
humansex cell
diploid (2n)
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
4 sperm cells are produced from each
primary spermatocyte.
Primary Spermatocyte
Secondary Spermatocyte
Secondary Spermatocyte
n=23
n=23
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Oogenesis
*** The polar bodies die… only one ovum (egg) is produced from each primary oocyte.
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Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.
• Centriole pairs also replicate.
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Interphase I
• Nucleus and nucleolus visible.
nuclear membrane
nucleolus
cell membrane
chromatin
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Meiosis I (four phases)
• Cell division that reduces the chromosome number by one-half.
• four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
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Prophase I
• Longest and most complex phase.• 90% of the meiotic process is spent in Prophase I
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes come together to form a tetrad.
• Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).
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Prophase I - Synapsis
Homologous chromosomes
sister chromatids sister chromatidsTetrad
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During Prophase I “Crossing Over” occurs.
• During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. The Chiasmata
(chiasma) are the sites of crossing over.
Crossing Over is one of the Two major occurrences of Meiosis
(The other is Non-disjunction)
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Crossing Over creates variation (diversity) in the offspring’s traits.
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
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Meiosis Makes Lots of Different Sex Cells Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent
assortment.
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Prophase I
centriolesspindle fiber
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Metaphase I• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n
Example: 2n = 4then n = 2
thus 22 = 4 combinations
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Question:
• In terms of Independent Assortment -how many different combinations of sperm could a human male produce?
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Answer
• Formula: 2n
• Human chromosomes: 2n = 46
n = 23
• 223 = ~8 million combinations
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Metaphase I
Independent assortment
As the chromosomes are pushed around during prophase I, eventually lining up along the metaphase plate during metaphase I, their positioning is different from that of mitosis metaphase.
Instead of lining one on top of the other, the replicated chromosomes line up side by side according to their homologous characterstics.
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Metaphase I
metaphase plate
OR
metaphase plate
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Anaphase I
• Homologous chromosomes separate and move towards the poles.
• Sister chromatids remain attached at their centromeres.
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Anaphase I
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Telophase I
• Each pole now has haploid set of chromosomes.
• Cytokinesis occurs and two haploid daughter cells are formed.
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Telophase I
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Meiosis II
• No interphase II
(or very short - no more DNA replication)
• Remember: Meiosis II is similar to mitosis
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Prophase II
• same as prophase in mitosis
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Metaphase II
• same as metaphase in mitosis
metaphase platemetaphase plate
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Anaphase II
• same as anaphase in mitosis• sister chromatids separate
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Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember: four haploid daughter cells produced.
gametes = sperm or egg
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Telophase II
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Non-disjunction
• Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to
separate during meiosis.• Non-disjunction results with the production
of zygotes with abnormal chromosome numbers…… remember…. An abnormal
chromosome number (abnormal amount of DNA) is damaging to the offspring.
Non-disjunction is one of the Two major occurrences of Meiosis
(The other is Crossing Over)
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Non-disjunctions usually occur in one of two fashions.
• The first is called Monosomy, the second is called Trisomy. If an
organism has Trisomy 18 it has three chromosomes in the 18th set, Trisomy 21…. Three chromosomes in the 21st
set. If an organism has Monosomy 23 it has only one chromosome in the 23rd
set.
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Common Non-disjunction Disorders
• Down’s Syndrome – Trisomy 21• Turner’s Syndrome – Monosomy 23 (X)• Kleinfelter’s Syndrome – Trisomy 23 (XXY)• Edward’s Syndrome – Trisomy 18
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Amniocentesis
• An Amniocentesis is a prrocedure a pregnant woman can have in order to
detect some genetics disorders…..such as non-disjunction.
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Amniocentesis
Amniotic fluid withdrawn
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REVIEW
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