4. Cellular Sexual Reproduction

8/12/2019 4. Cellular Sexual Reproduction

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Advantages of Sexual

Reproduction

Sexual reproduction allows the shuffling of genes

Sexual reproduction may combine different

parental alleles to produce a genetically unique

offspring

Sexual reproduction allows the combining

genetically determined traits, leading to rapid

evolution

The rate of mutations is very low, although they

produce new alleles


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Chromosomes Exist in

Homologues

Somatic cellshave homologous pairs ofchromosomes, with each chromosome of a pair

coming from each parent

Homologous chromosomescode for the same

genes, but have different alleles of each gene,

allowing for different expressions of the same

gene

Homologous pairs are matched in:

Length

Centromere position

Gene loci: different alleles of the same gene are

found at the same gene loci on the maternal andpaternal chromosomes


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Cells of Reproduction

Diploid cellshave homologous pairs

Haploid cellsdo not have homologous pairs, and

are formed through meiosis

Gametesare haploid cells produced by meiosisin sex organs

Sperm: male

Egg: female

Fertilizationis the union of sperm and egg Zygotes are formed by fertilization and are

diploid, because they are the fusion of two

haploid gametes


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Meiosis Meiosisis the process of cell division that converts

one diploid cell into four haploid cells

The basic cellular mechanisms for meiosis are the

same as mitosis

The events of Interphase are the same as in mitosis G1: growth and development

S: DNA duplication

G2: preparation for cell division

Meiosis is has two cell divisions, unlike mitosis In Meiosis I, homologous chromosomes separate

Forms two haploid cells from diploid cells

In Meiosis II, sister chromatids separate

Chromosome number remains the same and two haploid cells

result in four haploid cells


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Meiosis

Meiosis is divided into two stages: Meiosis I: separation of homologous chromosomes

Meiosis II: separation of sister chromatids

Each stage of meiosis is further divided into

prophase, metaphase, anaphase, telophase, and

cytokinesis (I or II)


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2n

n x

2

n nnn

2n x

2

n x

2

Interphase: Cell Growth

and DNA Duplication

Meiosis I:

Separation of

Homologues

Meiosis II:

Separation

of Sister

Chromatids

Diploid

Haploi

d

Diploid

Haploi

d


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Meiosis I: Prophase I Prophase I is the first stage of Meiosis I and is the

longest

The main purpose of Prophase I is to pair uphomologues and have them cross over

Prophase I is further divided into 5 stages: Leptotene

Zygotene

Pachytene

Diplotene Diakinesis

These subdivisions are described as theoccurrences that happen that are visibleunder an

electron microscope


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Prophase I: Leptotene and

Zygotene

In leptotene, the first stage of Prophase I, thecondensation of chromosomes is clearly visible

under the electron light microscope, as paired

sister chromatids

In zygotene, the second stage of Prophase I,

homologous pairs are paired one on top of the

other, in a process called synapsis

The synaptonemal complex (SC)are lateral

protein filaments, mostly cohesin, that hold

homologous pairs together; they are not completely

formed untilpachytne

The resulting structure after synapsis is called

tetrad or bivalent


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Prophase I: Pachytene In pachytene, the third stage of Prophase I, the SC is

completely formed and crossing over occurs Crossing over, also called genetic recombination, is the

process where the certain segments of the same geneloci are exchanged between the non-sister chromatids of

the maternal and paternal chromosomes The site of crossing over forms a covalent junction

where the crossing over sites the chromosomeintertwine and are held together using cohesin, calledthe chiasmata

Crossing over creates new allelic combinations, and theresulting gametes are potentially valuable in theevolution of an organism

Crossing over ensures that no pure maternal or paternalchromosomes exist within humans


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Meiosis I: Metaphase I Metaphase I is the second stage of meiosis Mitotic spindles attach to the kinetochores of the tetrads

The homologous chromosomes of the tetrads attach to thespindles of opposite poles

The sister chromatids of the same chromosome attach to thespindles of the same poles

Mitotic spindles align the tetrads along the metaphaseplate

The orientation of the paternal and maternal homologuesare random, meaning that not all the maternal or paternalchromosomes face the same pole, but rather a mix of

paternal and maternal chromosomes face either poles This means that the nucleus of the daughter cell after

Meiosis I is a mixture of maternal and paternalchromosomes that was randomly separated from eachhomologue

This process is called independent assortment, andproduces genetic variability. The combinations of

n


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Meiosis I: Anaphase I

Anaphase I is the third stage of meiosis

The purpose of Anaphase Iis to separate the

homologous pairs so that the daughter cells can

receive half of each pair

In order for the mitotic spindles to easily pull the

homologues away from each other, the cohesin of

the chiasmata are proteolyzed

After the dissolution of the cohesin, the mitoticspindles split the homologues apart from each

other and each chromosome is pulled to an

opposite pole

The sister chromatids of each chromosome


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Meiosis I: Telophase I and

Cytokinesis

During Telophase I, the fourth stage of mitosis: Chromosomes decondense Nuclear envelope reforms Nucleolus reforms

However, the events of Telophase I are usually less

dramatic than in the Telophase of mitosis, since thedaughter cells need to divide again anyways This means that the chromosomes do not fully

decondense and the nuclear envelope and nucleolusmay or may not reform

Cytokinesis in meiosis uses the same mechanism asin mitosis, using cleavage furrows to pull themembrane in

The two resulting daughter cells are haploid becausethey do not contain homologous pairs, but still have

double the DNA because each chromosome still hasa sister chromatid


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Meiosis II The main purpose of Meiosis II is to separate of sister

chromatids

The mechanism for Meiosis II is very similar to that ofmitosis

Prophase I: chromosomes recondense, nuclearenvelope and nucleolus break down if they evenreformed during Telophase I

Metaphase II: the kinetochores of sister chromatidsattach to mitotic spindles of opposite poles

Anaphase II: the mitotic spindles split sisterchromatids apart and pull them to opposite poles

Telophase II: the chromosomes decondense, nuclearenvelope and nucleolus forms, mitotic spindles

disassemble C tokinesis: the cell membrane is ulled in usin a


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End Result of Meiosis

From one parent cell before Meiosis I to the endof Meiosis II, four daughter cells are formed

Each daughter cell contains one full set of DNA,

but does not have homologous chromosomes,

and is therefore a haploid

To produce gametes, haploid cells have to go

through differentiation


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Mitosis vs. Meiosis

Mitosis Meiosis

Number of chromosomal

duplications

1 1

Number of cell divisions 1 2

Number of daughter cells

produced

2 4

How chromosomes line up during

metaphase plate

Homologues

individually

Homologues

together

Genetic relationship of daughter

cells to parent cell

Identical Non-identical

Functions performed in the human

body

Growth, repair Spermatogenesis

and oogenesis


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Gamete Fusion and Genetic

Variability

Two gametes of opposite sexes fuse to create adiploid zygote

The fusion of gametes add further geneticvariability to the offspring

Each gamete by independent assortment alonecan have 2n possible combinations of its parentschromosomes

Therefore, the fusion of two gametes will create

2n x 2n combinations of chromosomes The gametes from two humans could produce

about 64 trillion different combinations

This process is beneficial for the existence andwell-being as a species, as it makes it morediverse

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4. Cellular Sexual Reproduction