Download Reproduction Asexual or Sexual? What are we learning? SB2 c

Reproduction
Asexual or Sexual?
What are we learning?
 SB2 c. Using Mendel’s laws, explain the role of meiosis in reproductive variability.
 SB2 e. Compare the advantages of sexual reproduction and asexual reproduction in different
situations.
Asexual Reproduction
 Binary fission – when one cell splits into two cells.
◦ DNA replicates during interphase
◦ In Eukaryotic cells the nucleus divides during during mitosis
◦ The cell divides during cytokinesis
Mitosis – the division of the nucleus
 Prophase – DNA becomes visible and is called chromosomes, the nuclear envelope dissolves
 Metaphase – the chromosomes meet in the middle of the cell
 Anaphase – the sister chromosomes split and move towards the poles
 Telophase – the chromosomes unwind, and the nuclear membrane reforms
Other uses for Cell division
 Used to repair tissues
 For growth.
Advantages and Disadvantage of Asexual Reproduction
 Advantages
◦ Only needs one organism to reproduce
◦ Make lots of individuals quickly
◦ Are identical to parents (keeps the good adaptations that helps survival)
 Disadvantages
◦ Does not allow for mutations
◦ Can not adapt to the changes in the environment
Sexual Reproduction
 When DNA is exchanged between two organisms
 Two ways to do this
◦ Conjugation - the exchange of genetic material
◦ Exchange of sex cells
Exchange of sex cells
 In higher plants and animals
◦ Female – eggs
◦ Male – sperm
 In lower organism
◦ + and – are used to denote sex cells
How are sex cells made?
 Through the process of meiosis
◦ The process make haploid (n) cells (gametes)
▪ Females – 1 cell is the result (the other three are called polar bodies, they are not used
for reproduction)
▪ Males – 4 cells are produced
Haploid? Diploid?
 Contains only a single set of chromosomes or single set of genes.
◦ Remember you get one set of genes from your mother the other set from your father.
 Diploid-contains both sets of chromosomes.
Okay, so what are the stages of Meiosis?
 Meiosis I
◦ Prophase I
◦ Metaphase I
◦ Anaphase I
◦ Telophase I
 Meiosis II
◦ Prophase II
◦ Metaphase II
◦ Anaphase II
◦ Telophase II
Prophase I
 Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
 During prophase I homologous chromosomes may cross over and exchange portions of their
chromatids. This process is called crossing over.
What is a homologous chromosome?
 Two chromosomes that make up a matched pair in a diploid cell. One homologous
chromosome is inherited from the organism’s mother, the other from the organism’s father.
What is a tetrad?
 A paired set of homologous chromosomes, each composed of two sister chromatids.
Metaphase I
 Spindle fibers attach to the chromosomes and tetrads line up in the center of the cell.
Anaphase I
 The fibers pull the homologous chromosomes toward opposite ends of the cell.
Telophase I
 Nuclear membranes form. The cell separates into two haploid cells.
Cytokinesis I
 Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes
as the original cell.
Prophase II
 The DNA coils back up around the proteins forming chromosomes.
 The nuclear membrane dissolves
 The spindle fibers reform
Metaphase II
 The chromosome line up in a similar way to the metaphase stage of mitosis.
Anaphase II
 The sister chromatids separate and move toward opposite ends of the cell.
Telophase II
 Meiosis II results in four genetically different haploid (N) cells.
Advantages and Disadvantages of Sexual Reproduction

Advantages
◦ Organism can adapt to changes in the environment
◦ Increase variability in the gene pool
 Disadvantages
◦ Takes two
◦ Takes more time
◦ Typically less offspring
Accidents that can occur during Meiosis
 Nondisjunction – Members of a chromosome pair fail to separate during anaphase. This results
in diploid gametes. Crossing over, that sometimes occur during Prophase I, can cause this.
 Examples: Trisomy 21(Down’s syndrome) and Klinefelter syndrome.
 What alterations of chromosome structure (mutations) could lead to birth defects and cancer?
 Deletion- When a fragment of a chromosome is lost.
 Duplication- When a fragment from one chromosome joins to a homologous chromosome.
 Inversion- When a fragment reattaches to the original chromosome but in the reverse direction.
 Translocation- the attachment of a chromosomal fragment to a nonhomologous chromosome.
Quiz:
1. Name 2 ways in which
meiosis differ from mitosis?
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2.What is a gamete?
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3.What does haploid mean?
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4.If a human skin cell contains
46 chromosomes, how many
chromosomes would a human
sperm cell contain?
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5.If a human skin cell contained
46 chromosomes, how many
chromosomes would a human
liver cell contain?
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6. What types of accidents can
occur in Meiosis?
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7.What is crossing over? When
does it occur?
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8. Name and describe one alteration ___________________________________________________
of chromosome structure.
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