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BIOLOGY 101
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
• 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
OVERVIEW: Meiosis and Sexual Life Cycles
• 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 critical function of Meiosis is to increase genetic diversity
o Genetic diversity is the cornerstone of natural selection and evolution
The Cell Cycle:The Key Roles of Cell Division
Can you explain how the 2 primary purposes of meiosis occur?
13.1 Offspring acquire genes from their parents by inheriting chromosomes
• 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
The Cell Cycle:Background: Genes and Inheritance
• Images of the 46 human chromosomes can be arranged in pairs in order of size to produce a karyotype display.
o There are 46 chromosomes because each parent donates a set of 23 – meaning that we have 2 of each chromosome
o The two chromosomes in such a set, or homologous pair, have the same length, centromere position, and staining pattern.
13.1 Offspring acquire genes from their parents by inheriting chromosomes
The Cell Cycle:Background: Genes and Inheritance
• 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 Chromosomeso 22 human chromosome pairs are
homologous – these are called autosomeso The 23rd pair, the sex chromosomes (X and
Y), are not homologous
13.1 Offspring acquire genes from their parents by inheriting chromosomes
The Cell Cycle:Background: Genes and Inheritance
• 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).
13.1 Offspring acquire genes from their parents by inheriting chromosomes
The Cell Cycle:Background: Genes and Inheritance
• 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 homologous chromosomes
Two Sister Chromatids
Two Homologous Chromosomes
13.1 Offspring acquire genes from their parents by inheriting chromosomes
The Cell Cycle:Background: Genes and Inheritance
• 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 (22+1)
pairs of homologous chromosomes
• Meiosis separates homologous chromosomes and sister chromatids, leaving 4 new cells with only one set (23) chromosomes
Two Sister Chromatids
Two Homologous Chromosomes
13.1 Offspring acquire genes from their parents by inheriting chromosomes
The Cell Cycle:Background: Genes and Inheritance
• 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).
13.1 Like begets like, more or less: a comparison of asexual and sexual reproduction
• 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
The Cell Cycle:Offspring acquire genes from their parents by inheriting chromosomes
• 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
13.2 Human cells contain sets of chromosomes
The Cell Cycle:Fertilization and meiosis alternate in sexual life cycles
• 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 setof 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.
13.2 Let’s discuss the behavior of chromosome sets in the human life cycle
The Cell Cycle:Human cells contain sets of chromosomes
• 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
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.
The Cell Cycle:Human cells contain sets of chromosomes
• 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 both the maternal and paternal 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.
The Cell Cycle:Human cells contain sets of chromosomes
• 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
This is why gametes cannot be produced using mitosis!
The process of Meiosis is how gametes are produced
Meiosis starts with a diploid cell and ends with haploid gametes
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.
The Cell Cycle:Human cells contain sets of chromosomes
o Human sperm or ova have a haploid set of 23 different chromosomes, one from each homologous pair.
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.
The Cell Cycle:Human cells contain sets of chromosomes
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
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
The Cell Cycle:The Key Roles of Cell Division
• Many steps of meiosis resemble steps in mitosis.
• Both meiosis and mitosis are preceded by the duplication of chromosomes.
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
The Cell Cycle:The Key Roles of Cell Division
• 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.
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
The Cell Cycle:The Key Roles of Cell Division
• 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.
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
The Cell Cycle:The Key Roles of Cell Division
• Note that in Metaphase I of Meiosis, it is the homologous chromosomes that align!
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.
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
• 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
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
Meiosis Overview:
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 (also genetic variation)
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
• Meiosis I consists of Prophase I, Metaphase I, Anaphase I and Telophase I
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 (also genetic variation)
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
• Meiosis I consists of Prophase I, Metaphase I, Anaphase I and Telophase I
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 (also genetic variation)
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
• 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
13.3 We can compare mitosis and meiosis
• Four 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
4. The generation of 2 new haploid cells in telophase I
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
13.3 Crossing over and synapsis occurs during prophase I.
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
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)
13.3 Crossing over and synapsis occurs during prophase I.
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
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.
13.3 Crossing over and synapsis occurs during prophase I.
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
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
13.3 We can compare mitosis and meiosis
• Four 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
• Four events unique to meiosis occur during meiosis I:
2. Random Homolog Alignment
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
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 calledIndependent Assortment, and increases genetic diversity
13.3 We can compare mitosis and meiosis
• Four events unique to meiosis occur during meiosis I:
2. Random Homolog Alignment
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
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
13.3 We can compare mitosis and meiosis
• Four events unique to meiosis occur during meiosis I:
3. Separation of parental homologs
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
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
13.3 We can compare mitosis and meiosis
• Four events unique to meiosis occur during meiosis I:
3. Separation of parental homologs
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
o During telophase I of meiosis, cytokinesis also occurs, creating two new haploid cells
✓ The two haploid cells still have sister chromatids, so those must be separated in meiosis II
✓ Nuclear membranes briefly reform, and spindles briefly disassemble
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
Meiosis II Overview:
The purpose of Meiosis II is to separate Sister Chromatids – just like in Mitosis!
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 II Overview:
The purpose of Meiosis II is to separate Sister Chromatids – just like in Mitosis!
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 II Overview:
The purpose of Meiosis II is to separate Sister Chromatids – just like in Mitosis!
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
• Prophase II: Nuclear membranes breakdown and spindles reassemble
• Metaphase II is when sister chromatids line up at the metaphase plate
• Anaphase II is when sister chromatids separate
• Telophase II produces 4 new haploid cells
✓ Nuclear membranes reform, spindles disassemble and chromosomes unpack
13.3 Meiosis reduces the number of chromosome sets from diploid to haploid
Meiosis II Overview:
The purpose of Meiosis II is to separate Sister Chromatids – just like in Mitosis!
The Cell Cycle:Meiosis reduces the number of chromosome sets from diploid to haploid
• Prophase II: Nuclear membranes breakdown and spindles reassemble
• Metaphase II is when sister chromatids line up at the metaphase plate
• Anaphase II is when sister chromatids separate
• Telophase II produces 4 new haploid cells
✓ Nuclear membranes reform, spindles disassemble and chromosomes unpack
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:Meiosis reduces the number of chromosome sets from diploid to haploid
13.4 Genetic variation produced in sexual life cycles contributes to evolution
• Meiosis is one mechanism to increase genetic diversity (creates new combinations of alleles)
• 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:The Key Roles of Cell Division
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.
The Cell Cycle:Genetic variation produced in sexual life cycles contributes to evolution
• Independent Assortment: 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.
Crossing Over
Independent Assortment
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.
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.
• A population evolves through the differential reproductive success of its variant members.
o 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. More variation comes from crossing over and independent assortment
• 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.
The Cell Cycle:Genetic variation produced in sexual life cycles contributes to evolution