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Chapter 8-2 – Sexual Reproduction & Meiosis -

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1 Chapter 8-2 – Sexual Reproduction & Meiosis -
General Biology (Bio107) Chapter 8-2 – Sexual Reproduction & Meiosis -

2 Reproduction & Role of meiosis
Reproduction is one of the hallmark characteristics of all forms of life on planet Earth; all living organisms reproduce During reproduction they hand over (= inherit) their individual genetic make-up and information over to a next generation The transmission of traits from one generation to the next is called heredity or inheritance. Reproduction assures the continuation of a species over time Reproduction means the formation of a new organism from a pre-existing one - for most organisms the new individual starts with a fertilized egg in a process called fertilization

3 Types of Reproduction Three major forms of reproduction are established in the living world. 1. Sexual reproduction - predominant form of reproduction in multi-cellular eukaryotes and animals fusion of a haploid egg cell and one haploid sperm forms diploid zygote which starts a new individual 2. Asexual reproduction - type of reproduction mostly found in bacteria and amoeba - e.g. bacteria reproduce in process called ‘binary fission’ DNA of the offspring is inherited from one parent or cell and is identical with parental DNA 3. Parthenogenesis - rare form of reproduction is performed by certain amphibians and reptiles - e.g. whiptail lizard

4 Common observations with sexual reproduction: 1
Common observations with sexual reproduction: 1. Offspring resemble their parents more than they do less closely related individuals of the same species 2. Offspring differ somewhat from parents and siblings, demonstrating variation. 3. Parents endow their offspring with coded information in the form of genes 4. Offspring of sexual reproduction vary genetically from their siblings and from both parents.

5 Alternation of haploid (1n) and diploid (2n)
stages in the human life cycle Gonads Ovaries Testis Zygote (2n) Oocyte (egg cell) Gametes (1n) Sperm 23 chromosomes (= 1n) 23 chromosomes (= 1n)

6 Genes Genes program specific traits that emerge as we develop from fertilized eggs into adults Genes are segments of DNA Genetic information is transmitted as specific sequences of the four deoxyribonucleotides in DNA Cells translate genetic “sentences” (= nucleotide sequences) into traits and other features with no resemblance to genes Most genes program cells to synthesize specific enzymes and other proteins that produce an organism’s inherited traits.

7 Chromosomes Almost all of the DNA in a eukaryotic cells is subdivided into chromosomes in the nucleus. Tiny amounts of DNA are found in mitochondria and chloroplasts Every living species has a characteristic number of chromosomes. Humans have 46 chromosomes Each chromosome consists of single DNA molecule in association with various proteins. Each chromosome has hundreds or thousands of genes, each at a specific location, it’s locus.

8 Chromosome Sets Sets of paternal and maternal chromosomes form the homologous chromosome pairs The complete set of chromosomes of an organism is called the diploid number Chr #1 – 22: Autosomes Sex or gender chromosomes

9 Gamete formation & Role of meiosis
Early during embryogenesis, haploid gametes are formed by specialized cells, the primordial germ line cells, by a unique form of cell division called meiosis Meiosis involves reduction of the genetic material from a double (= diploid, 2n) chromosomal set to a single (= haploid, 1n) set Meiosis comprises two successive nuclear divisions with only one round of DNA replication Meiosis creates: 1. Haploid germ cells (eggs and sperm) from a diploid parent cell 2. Increased genetic variety due to crossing over events

10 The two cell division phases of meiosis
FSH stimulates One Primordial Germ cell (2n)  In gonads 4 Gametes (1n)

11 The first division, meiosis I, separates homologous chromosomes.
Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice. The first division, meiosis I, separates homologous chromosomes. The second, meiosis II, separates sister chromatids. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

12 Interphase Division in meiosis I occurs in four phases: prophase, metaphase, anaphase, and telophase. During the preceding interphase the chromosomes are replicated to form sister chromatids. These are genetically identical and joined at the centromere. Also, the single centrosome is replicated.

13 Prophase I In prophase I, the chromosomes condense and homologous chromosomes pair up to form tetrads. In a process called synapsis, special proteins attach homologous chromosomes tightly together. At several sites the chromatids of homologous chromosomes are crossed (chiasmata) and segments of the chromosomes are traded. A spindle forms from each centrosome and spindle fibers attached to kinetochores on the chromosomes begin to move the tetrads around.

14 New allele combinations due to crossing over in Prophase I
maternal a b c allele paternal a b c EM Image a b c a b c

15 Metaphase I At metaphase I, the tetrads are all arranged at the (virtual) metaphase plate. Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad, while those from the other pole are attached to the other. In anaphase I, the homologous chromosomes separate and are pulled toward opposite poles.

16 8 different Metaphase I Alignments are possible with 6 chromosomes
Random alignment of homologous chromosomes in Metaphase I General: Haploid number n = 3 Ncomb = 2n DNA-replication For this example: Prophase I Ncomb = 23 = 8 or or or or or or or or 8 different Metaphase I Alignments are possible with 6 chromosomes

17 Telophase I & Cytokinesis I
In telophase I, movement of homologous chromosomes continues until there is a haploid set at each pole. Each chromosome consists of linked sister chromatids. Cytokinesis by the same mechanisms as mitosis usually occurs simultaneously. In some species, nuclei may reform, but there is no further replication of chromosomes.

18 Prophase II Meiosis II is very similar to mitosis, but no DNA replication takes place at the beginning of meiosis II During prophase II a spindle apparatus forms, attaches to kinetochores of each sister chromatids, and moves them towards the metaphase plate. Spindle fibers from one pole attach to the kinetochore of one sister chromatid and those of the other pole to the other sister chromatid.

19 Metaphase II & Anaphase II
At metaphase II, the sister chromatids are arranged at the (virtual) metaphase plate. The kinetochores of sister chromatids face opposite poles. At anaphase II, the centomeres of sister chromatids separate and the now separate sisters travel toward opposite poles.

20 Telophase II & Cytokinesis II
In telophase II, separated sister chromatids arrive at opposite poles. Nuclei form around the chromatids. Cytokinesis II separates the cytoplasm. At the end of meiosis, there are four haploid daughter cells which are genetically different

21 Mitosis produces two identical daughter cells, but meiosis produces 4 very different cells.

22 of homologous chromosomes Metaphase I
Activation by FSH Diploid (2n) Gamete mother cell DNA-replication Tetrad formation & Crossing over Prophase I Random alignment of homologous chromosomes Metaphase I Homologous chromosomes segregate Anaphase I Prophase II Sister chromatids separate Anaphase II Haploid (1n) Gametes (sperm or oocytes)

23 Meiosis & Non-disjunction events
Meiosis consists of many intricate steps, involving many enzymes and protein components It is a highly vulnerable cellular process prone to errors Errors which affect the symmetric separation of chromosomes during meiosis I or II lead to alterations of chromosome numbers or chromosomal aberrations Following accidents during meiosis leading to alterations of chromosome numbers are: 1. Non-disjunction in meiosis I  homologous chromosome pairs fail to separate during Metaphase I of meiosis  n+1 or n-1 gametes form at the end of meiosis 2. Non-disjunction in meiosis II  sister chromatids fail to separate during Metaphase II  n, n+1 or n-1 gametes form at the end of meiosis

24 Aberrant chromosome numbers in gametes due to Non-disjunction event in meiosis I
Inter- phase Pro- phase I Meta- phase II n+1 or n-1 Gametes Disturbing Factor Graphic©E.Schmid/2001

25 Aberrant chromosome numbers in gametes due to Non-disjunction event in meiosis II
Inter- phase Pro- phase I Meta- phase II n, n+1 or n-1 Gametes Disturbing Factor Graphic©E.Schmid/SWC2001 Normal

26 Fertilization involving gametes with wrong (= aberrant) chromosome numbers or chromosome patterns results in offspring with chromosomal abnormalities, = additional/extra or missing chromosomes in the resulting zygote and embryo The most common chromosomal aberrations in humans are: 1. Trisomies cells have extra chromosome 2. Monosomies cells are missing one chromosome Severe chromosomal aberrations or defects in an individual or embyo can be early detected and analyzed by preparing a so-called karyogram

27 Chromosomal aberration & Karyotype analysis
Karyotypes, ordered displays of an individual’s chromosomes, are often prepared with lymphocytes. Normal Karyogram

28 A typical karyotype analysis involves comparing chromosomes for their:
1. Number (e.g. Trisomies) 2. Length (e.g. deletions, translocations) 3. Placement of centromeres 4. Location and sizes of chromosomal G-bands (deletions, insertions, inversions)  the Giemsa dye stains regions of chromosomes that are rich in the base pairs Adenine (A) and Thymine (T) where it produces the typical dark bands, the G-bands Human chromosome pair

29 Normal karyogram of a human female
2 3 4 5 6 7 1 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX Sex chromosomes Chr. 1  22 = Autosomes

30 Genetic disorders in humans caused by chromosomal aberrations
1. Trisomy 21 (= Down syndrome) - Cells show extra copy of the chromosome 21 in a karyogram - Together with Trisomy 13 - the most common chromosome number abnormality in humans - Affects about 1 out of every 700 children born in the US - Affected people suffer from heart defects, susceptibility to respiratory infections, leukemia; have a shorter life-span - Many affected individuals exhibit varying degrees of mental retardation - Incidence of Down syndrome in the offspring of genetically normal parents increases markedly with the age of the mother (> 35 y old) - The chromosome alterations of Trisomy 21 are suspected to occur after fertilization

31 Trisomy 21 Karyogram

32 2. Trisomy 13 (Pateu’s syndrome)
Affected human individuals are characterized by cells which show an extra copy of the chromosome 13 in a karyogram Chromosomal aberration is observed in about 1/5000 life births Trisomy 13 babies are frequently stillborn or die as newborns Physical abnormalities include: severe mental retardation, growth retardation, mis-development of the brain/spinal cord, cleft lip and palate, cyclopia (one eye) often with protruding facial proboscis Trisomy 13 Karyogram

33 Examples of clinical features of Trisomy 13:

34 Chromosomal aberrations of sex chromosomes
Non-disjunction events affecting sex chromsosomes - have usually less dramatic effects on the genetic balance and on phenotype of the carrier  probably due to lesser number of genes on sex chromosomes and natural inactivation of the second X-chromosome in an XX (= female) individual The most common sex chromosome abnormalities are: 1. Extra Y chromosome (XYY) - Observed in 1/2500 live births - Affected males have larger body stature and borderline intelligence - New studies give hints to mild to severe social behavioral disturbances with trend to accumulated criminal records 2. XXY = Klinefelter syndrome) - Cells of human males show an extra X chromosome - Observed in 1/2000 live births - Affected males have abnormally small testes & sterility - Often accompanied by breast enlargement

35 Chromosomal aberrations of sex chromosomes
3. XXX = Multi-X female (“super-female”) - Carriers have limited fertility - Around 1 in 1000 woman has three or more X-chromosomes - Most 47,XXX women are phenotypically ‘normal’ - 48,XXXX woman are usually mildly retarded, and 49,XXXXX produces severe disability 4. XO = Turner syndrome - Affected individuals are females - Show underdeveloped ovaries, poor breast development and a so-called “web of skin” between neck and shoulders - Carriers are sterile

36 Other Chromosomal Changes
More subtle changes in chromosome structures and shapes can be caused by a series of other factors, such as strong irradiation (e.g. X-rays, radioactivity) and environmental factors (e.g. toxins, viruses). Events which lead to abnormal chromosomal structures are: 1. Deletions - A chromosomal piece breaks off and gets lost Chromosome 5p deletion syndrome or (“cat cry syndrome”) - caused by deletion of the short (= p) arm of the human Chr # 5 - affects 1/20,000 to 1/50,000 human life births - infants with cri du chat syndrome commonly have a distinctive cat-like cry - show severe mental retardation, low birth weight, microcephaly, webbed fingers or toes Chromosome 13q deletion - common finding in human blood cancers, such as: B-cell chronic lymphocytic leukemia (CLL), non-Hodgkin's lymphoma

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