Download Lecture 14 Notes CH.13

BIOLOGY 101
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CHAPTER 13: Meiosis and Sexual Life Cycles:
Variations on a Theme
Meiosis and Sexual Life Cycles:
Variations on a Theme
CONCEPTS:
•
13.1 Offspring acquire genes from their parents by inheriting chromosomes
•
13.2 Fertilization and meiosis alternate in sexual life cycles
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13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
•
13.4 Genetic variation produced in sexual life cycles contributes to evolution
The Cell Cycle:
The Key Roles of Cell Division
OVERVIEW: Meiosis and Sexual Life Cycles
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The primary purpose of Meiosis is to create haploid gametes, or egg and sperm that have only 1
pair of 23 chromosomes instead of 2 pairs of 23 chromosomes (diploid)
•
Another purpose of Meiosis is to increase genetic diversity
o
Genetic diversity is the cornerstone of natural selection and evolution
Can you explain how each of these occur?
The Cell Cycle:
Background: Genes and Inheritance
13.1 Offspring acquire genes from their parents by inheriting chromosomes
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Parents endow their offspring with coded information in the form of genes.
o
Your genome is made up of the genes that you inherited from your mother and your father.
•
Genes program specific traits that emerge as we develop from fertilized eggs into adults.
•
Genes are segments of DNA.
The Cell Cycle:
Background: Genes and Inheritance
13.1 Offspring acquire genes from their parents by inheriting chromosomes
•
•
In plants and animals, reproductive cells called gametes (egg and sperm) transmit genes from one
generation to the next.
After fertilization (fusion of a sperm cell and an egg), genes from both parents are present in the
nucleus of the fertilized egg, or zygote.
The Cell Cycle:
Background: Genes and Inheritance
13.1 Offspring acquire genes from their parents by inheriting chromosomes
•
Genes exist on chromosomes and each gene has a specific location, or locus, on a specific chromosome
✓
For example: The hemoglobin gene is always on chromosome 11, and in the same location
✓
Humans have about 23,000 genes
The Cell Cycle:
Background: Genes and Inheritance
13.1 Offspring acquire genes from their parents by inheriting chromosomes
•
Images of the 46 human chromosomes can be
arranged in pairs in order of size to produce a
karyotype display.
o
The two chromosomes in a homologous
pair have the same length, centromere
position, and staining pattern.
The Cell Cycle:
Background: Genes and Inheritance
13.1 Offspring acquire genes from their parents by inheriting chromosomes
•
Somatic cells have two copies of each
chromosome
• One copy is paternal, coming from sperm
• One copy is maternal, coming from an egg
•
The pairs of chromosomes are called
Homologous Chromosomes
o 22 human chromosome pairs are
homologous – these are called autosomes
o The 23rd pair, the sex chromosomes (X and
Y), are not homologous
The Cell Cycle:
Background: Genes and Inheritance
13.1 Offspring acquire genes from their parents by inheriting chromosomes
•
Two distinct sex chromosomes, the X and the Y,
are an exception to the general pattern of
homologous chromosomes in human somatic
cells.
•
The pattern of inheritance of the sex
chromosomes determines an individual’s
gender.
o Human females have a homologous pair of
X chromosomes (XX); males have one X and
one Y chromosome (XY).
The Cell Cycle:
Background: Genes and Inheritance
13.1 Offspring acquire genes from their parents by inheriting chromosomes
Two Sister Chromatids
•
When cells reproduce, they replicate chromosomes
during the S-phase of Interphase – these copies of
chromosomes are called Sister Chromatids
•
During the cell cycle homologous chromosomes are
copied into Sister Chromatids
• In Mitosis, these sister chromatids are
separated, leaving each new cell with 23 pairs of
Two Homologous
homologous chromosomes
Chromosomes
The Cell Cycle:
Background: Genes and Inheritance
13.1 Offspring acquire genes from their parents by inheriting chromosomes
Two Sister Chromatids
•
When cells reproduce, they replicate chromosomes
during the S-phase of Interphase – these copies of
chromosomes are called Sister Chromatids
•
During the cell cycle homologous chromosomes are
copied into Sister Chromatids
• In Mitosis, these sister chromatids are
separated, leaving each new cell with 23 pairs of
Two Homologous
homologous chromosomes
Chromosomes
•
Meiosis separates homologous chromosomes
and sister chromatids, leaving 4 new cells with
only one set (23) chromosomes
The Cell Cycle:
Background: Genes and Inheritance
13.1 Offspring acquire genes from their parents by inheriting chromosomes
•
The occurrence of homologous pairs of
chromosomes is a consequence of sexual
reproduction.
o
We inherit one chromosome of each
homologous pair from each parent.
o
The 46 chromosomes in each somatic cell are
two sets of 23, a maternal set (from your
mother) and a paternal set (from your father).
The Cell Cycle:
Offspring acquire genes from their parents by inheriting chromosomes
13.1 Like begets like, more or less: a comparison of asexual and sexual reproduction
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Only organisms that reproduce asexually can produce offspring that are exact copies of them.
•
In asexual reproduction, a single individual is the sole parent to donate genes to its offspring.
o
An individual that reproduces asexually gives rise to a clone, a group of genetically identical
individuals.
o
Members of a clone may be genetically different as a result of mutation
The Cell Cycle:
Offspring acquire genes from their parents by inheriting chromosomes
13.1 Like begets like, more or less: a comparison of asexual and sexual reproduction
•
In sexual reproduction, two parents produce offspring that
have unique combinations of genes inherited from the two
parents.
o
Unlike a clone, offspring produced by sexual reproduction
vary genetically from their siblings and their parents
o
Genetic variation is the root of success for all living
organisms
o
Variation insures that there are traits in a population
that can survive a changing environment – this is the
basis of Natural Selection
The Cell Cycle:
Fertilization and meiosis alternate in sexual life cycles
13.2 Human cells contain sets of chromosomes
•
The number of chromosomes in a single set is
represented by n.
•
Any cell with two sets of chromosomes is called a
diploid cell and has a diploid number of
chromosomes, abbreviated as 2n.
o
Sperm cells or ova (gametes) have only one set
of chromosomes—22 autosomes and an X (in
an ovum) or 22 autosomes and an X or a Y (in a
sperm cell).
✓
A cell with a single chromosome set is a
haploid cell, abbreviated as n.
The Cell Cycle:
Human cells contain sets of chromosomes
13.2 Let’s discuss the behavior of chromosome sets in the human life cycle
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The human life cycle begins when a haploid sperm cell fuses
with a haploid ovum.
o
The union of these gametes, culminating in the fusion of
their nuclei, is fertilization.
•
The fertilized egg (zygote) is diploid because it contains two
haploid sets of chromosomes bearing genes from the maternal
and paternal family lines.
•
As a person develops from a zygote to a sexually mature adult,
mitosis generates all the somatic cells of the body.
o
Each somatic cell contains a full diploid set of
chromosomes
The Cell Cycle:
Human cells contain sets of chromosomes
13.2 Let’s discuss the behavior of chromosome sets in the human life cycle
•
QUESTION: What would happen if gametes were produced by mitosis? How many chromosomes would
each egg and sperm have?
•
ANSWER: If gametes were produced by mitosis, the fusion of gametes would produce offspring with
four sets of chromosomes after one generation, eight after a second, and so on.
•
This means that gametes cannot start out with
2 pairs (n=46) of each chromosome!
•
Gametes must become haploid (n=23),
containing only one set (23) of
chromosomes
•
The gametic set of 23 is a combination of
either the maternal or paternal
chromosomes
The Cell Cycle:
Human cells contain sets of chromosomes
13.2 Let’s discuss the behavior of chromosome sets in the human life cycle
•
QUESTION: What would happen if gametes were produced by mitosis? How many chromosomes would
each egg and sperm have?
•
ANSWER: If gametes were produced by mitosis, the fusion of gametes would produce offspring with
four sets of chromosomes after one generation, eight after a second, and so on.
•
This means that gametes cannot start out with
2 pairs (n=46) of each chromosome!
•
•
This is why gametes cannot be produced
using mitosis!
Gametes must become haploid (n=23),
containing only one set (23) of
chromosomes
The process of Meiosis is how gametes
are produced
The gametic set of 23 is a combination of
either the maternal or paternal
chromosomes
Meiosis starts with a diploid cell and
ends with haploid gametes
The Cell Cycle:
Human cells contain sets of chromosomes
13.2 Let’s discuss the behavior of chromosome sets in the human life cycle
•
Gametes, which develop in the gonads (testes or ovaries), are not produced by mitosis.
o
Gametes develop from specialized diploid cells called germ cells in the gonads.
o
Gametes undergo the process of meiosis, in which the chromosome number is halved.
o
Human sperm or ova have a haploid set of 23 different
chromosomes, one from each homologous pair.
The Cell Cycle:
Human cells contain sets of chromosomes
13.2 Let’s discuss the behavior of chromosome sets in the human life cycle
•
•
Gametes, which develop in the gonads (testes or ovaries), are not produced by mitosis.
o
Gametes develop from specialized diploid cells called germ cells in the gonads.
o
Gametes undergo the process of meiosis, in which the chromosome number is halved.
o
Human sperm or ova have a haploid set of 23 different
chromosomes, one from each homologous pair.
Fertilization restores the diploid condition by combining two
haploid sets of chromosomes
The Cell Cycle:
The Key Roles of Cell Division
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
•
Many steps of meiosis resemble steps in mitosis.
•
Both meiosis and mitosis are preceded by the duplication of
chromosomes.
The Cell Cycle:
The Key Roles of Cell Division
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
•
Many steps of meiosis resemble steps in mitosis.
•
Both meiosis and mitosis are preceded by the duplication of
chromosomes.
•
In meiosis, there are two consecutive cell divisions, meiosis I and
meiosis II, resulting in four daughter cells.
o
The first division, meiosis I, separates homologous
chromosomes.
o
The second division, meiosis II, separates sister chromatids.
✓
The four daughter cells at the end of meiosis have only half as
many chromosomes as the original parent cell.
The Cell Cycle:
The Key Roles of Cell Division
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
•
Many steps of meiosis resemble steps in mitosis.
•
Both meiosis and mitosis are preceded by the duplication of
chromosomes.
•
In meiosis, there are two consecutive cell divisions, meiosis I and
meiosis II, resulting in four daughter cells.
o
The first division, meiosis I, separates homologous
chromosomes.
o
The second division, meiosis II, separates sister chromatids.
✓
The four daughter cells at the end of meiosis have only half as
many chromosomes as the original parent cell.
The Cell Cycle:
Meiosis reduces the number of chromosome sets from diploid to haploid
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
•
Meiosis I is preceded by interphase, in which the chromosomes are duplicated to form sister chromatids
o
•
Two genetically identical sister chromatids make up one duplicated chromosome.
In contrast, the two chromosomes of a homologous pair are
individual chromosomes that were inherited from different
parents.
o
Homologous chromosomes appear to be alike, but they may
have different versions of genes, each called an allele, at
corresponding loci
The Cell Cycle:
Meiosis reduces the number of chromosome sets from diploid to haploid
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
Meiosis Overview:
One purpose of Meiosis I is to separate Homologous Chromosomes
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Meiosis I consists of Prophase I,
Metaphase I, Anaphase I and
Telophase I
One purpose of Meiosis II is to separate
Sister Chromatids
•
Meiosis II consists of Prophase II,
Metaphase II, Anaphase II and
Telophase II
The Cell Cycle:
Meiosis reduces the number of chromosome sets from diploid to haploid
13.3 Crossing over and synapsis occurs during prophase I.
PROPHASE I:
•
Crossing over begins very early in prophase I, as homologous
chromosomes pair loosely along their lengths.
o
•
Each gene on one homolog is aligned precisely with the
corresponding gene on the other homolog – process is called Synapsis)
In a single crossover event, the DNA of two nonsister chromatids of a
homologous pair, precisely align, forming a Tetrad
o
The two regions at the crossover point are joined to the other
chromatid.
o
Thus, a paternal chromatid is joined to a piece of maternal
chromatid, and vice versa.
✓
Crossing over creates genetic diversity because individual
chromosomes carry genes derived from two different parents
The Cell Cycle:
Meiosis reduces the number of chromosome sets from diploid to haploid
13.3 We can compare mitosis and meiosis
•
Three events unique to meiosis occur during meiosis I:
1.
Synapsis and crossing over in prophase I
2.
Random homolog alignment on the metaphase plate in metaphase I
3.
Separation of parental homologs in anaphase I
The Cell Cycle:
Meiosis reduces the number of chromosome sets from diploid to haploid
13.3 We can compare mitosis and meiosis
•
Three events unique to meiosis occur during meiosis I:
1.
Synapsis and crossing over
o
During prophase I, duplicated homologs pair up in a process known as synapsis – crossing over
occurs at this time
o
Synapsis and crossing over normally do not occur during prophase of mitosis.
The Cell Cycle:
Meiosis reduces the number of chromosome sets from diploid to haploid
13.3 We can compare mitosis and meiosis
•
Three events unique to meiosis occur during meiosis I:
2.
Random Homolog Alignment
o
During metaphase I of meiosis, 23 pairs of
homologs (rather than sister chromatids as in
mitosis) line up on the metaphase plate.
o
This alignment is random, with a mix of
maternal and paternal homologs on either
side
✓
This random alignment is called
Independent Assortment, and increases
genetic diversity
The Cell Cycle:
Meiosis reduces the number of chromosome sets from diploid to haploid
13.3 We can compare mitosis and meiosis
•
Three events unique to meiosis occur during meiosis I:
3.
Separation of parental homologs
o
During anaphase I of meiosis, the homologous chromosomes are
separated
o
The sister chromatids of each duplicated chromosome remain
attached.
✓
In anaphase of mitosis, by contrast, the sister chromatids are
separated
The Cell Cycle:
Meiosis reduces the number of chromosome sets from diploid to haploid
13.3 Genetic Variation is Generated During Meiosis I
•
Two events in Meiosis I create genetic diversity
1.
Crossing in prophase I (creates new versions of parental chromososmes)
2.
Independent Assortment in metaphase I (randomly separates parental homologs)
The Cell Cycle:
The Key Roles of Cell Division
13.4 Genetic variation produced in sexual life cycles contributes to evolution
•
Meiosis is one mechanism to increase genetic diversity
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Mutations are the original source of genetic diversity.
•
o
These changes in an organism’s DNA create the different versions of genes known as alleles.
o
Primitive life that did not use sexual reproduction primarily rely on mutation for their genetic
diversity
Once different versions of genes arise through mutation, reshuffling of alleles during sexual
reproduction results in each member of a species having a unique combination of traits
•
Sexual Reproduction is another tremendous increase in genetic diversity
The Cell Cycle:
Genetic variation produced in sexual life cycles contributes to evolution
13.4 Sexual life cycles produce genetic variation among offspring
•
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation
that arises in each generation.
•
Three mechanisms contribute to genetic variation arising from sexual reproduction:
Crossing over (prophase I)
Independent assortment of chromosomes (metaphase I)
Random fertilization (sexual reproduction)
The Cell Cycle:
Genetic variation produced in sexual life cycles contributes to evolution
13.4 Sexual life cycles produce genetic variation among offspring
•
Crossing over produces recombinant chromosomes, which combine genes inherited from each parent
•
During meiosis in humans, an average of one to three crossover events occur per chromosome pair
o
•
Crossing over, by combining DNA inherited from two parents into a single chromosome, is an
important source of genetic variation.
At metaphase I, nonidentical sister
chromatids sort independently from one
another, further increasing the number of
genetic types of daughter cells that are
formed by meiosis.
Independent
Assortment
Crossing Over
The Cell Cycle:
Genetic variation produced in sexual life cycles contributes to evolution
13.4 Sexual life cycles produce genetic variation among offspring
•
The random nature of fertilization adds to the genetic variation arising from meiosis
The Cell Cycle:
Genetic variation produced in sexual life cycles contributes to evolution
13.4 Evolutionary adaptation depends on a population’s genetic variation
Charles Darwin recognized the importance of genetic variation in evolution.
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A population evolves through the differential reproductive success of its variant members.
o
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Those individuals best suited (having the best allele combinations) to the local environment
leave the most offspring, transmitting their genes in the process.
This natural selection results in adaptation, the accumulation of favorable genetic variations in a
specific environment
The Cell Cycle:
Genetic variation produced in sexual life cycles contributes to evolution
13.4 Evolutionary adaptation depends on a population’s genetic variation
•
As the environment changes, the population may survive if some members can cope effectively with the
new conditions.
o Mutations are the original source of different alleles, which are then mixed and matched during
meiosis.
•
New and different combinations of alleles may work better than those that previously prevailed.
o
The ability of sexual reproduction to generate genetic diversity is one of the most commonly
proposed advantages to explain the evolutionary persistence of this process.
o
We will see later on that asexually reproducing organisms, such as bacteria, also have powerful
tools for genetic variation, but they do not come from meiosis or sexual reproduction