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CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES. INTRODUCTION TO HEREDITY HEREDITY- transmission of traits from one generation to the next GENETICS- the scientific.

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Presentation on theme: "CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES. INTRODUCTION TO HEREDITY HEREDITY- transmission of traits from one generation to the next GENETICS- the scientific."— Presentation transcript:


2 INTRODUCTION TO HEREDITY HEREDITY- transmission of traits from one generation to the next GENETICS- the scientific study of heredity and variation

3 Parents pass hereditary information to their offspring in the form of GENES - the tens of thousands of genes that we inherit from our fathers and mothers makes up our GENOME - it is this genetic link that accounts for family resemblance

4 Genes are segments of DNA: - inherited information is passed on in the form of each gene’s specific sequence of nucleotides - the cell translates these “genetic sentences” into features

5 How is heredity possible? - the transmission of hereditary traits depends on the precise replication of DNA - this produces copies of genes that can be passed on from parents to offspring Every living species has a characteristic number of chromosomes - humans have 46 chromosomes in almost all of their cells

6 - each chromosome contains thousands of genes - a gene’s specific location on a chromosome is called its LOCUS

7 ASEXUAL VS. SEXUAL REPRODUCTION ASEXUAL REPRODUCTION - a single individual is the SOLE parent and passes all of its genes to its offspring - some single-celled eukaryotes can reproduce asexually by mitotic cell division - some multicellular organisms (such as the Hydra) can reproduce asexually - binary fission is a form of asexual reproduction

8 - OFFSPRING ARE EXACT COPIES OF THE PARENT SEXUAL REPRODUCTION - two parents give rise to offspring that have unique combinations of genes inherited from the two parents - results in greater genetic variation - “like begets like” only in the sense of family resemblance

9 MEIOSIS AND SEXUAL LIFE CYCLES LIFE CYCLE- generation-to-generation sequence of stages in the reproductive history of an organism (from conception to the production of its own offspring)

10 Human Life Cycle SOMATIC CELL- any cell other than a sperm or egg - in humans, each somatic cell has 46 chromosomes - a KARYOTYPE is a picture of an organism’s chromosomes - when looking at a human karyotype, you can see that there are 2 of each type of chromosome

11 - these are called HOMOLOGOUS CHROMOSOMES  same length, centromere position, and staining pattern - the 2 chromosomes of each pair carry genes controlling the same inherited characteristics

12 X and Y chromosomes are an exception to the rule of homologous chromosomes: - human females are XX - human males are XY - only small parts of the X and Y are homologous (the Y is much smaller) - these are called SEX CHROMOSOMES because they determine an individual’s sex - all other chromosomes are called AUTOSOMES

13 The 46 chromosomes in our somatic cells are a result of the combination of chromosomes from our father and mother - we inherit 23 chromosomes from each parent Sperm cells and ova are called GAMETES - each of these cells has 22 autosomes plus 2 sex chromosomes - these are called HAPLOID CELLS because they have a single chromosome set

14 - abbreviated n (for humans, n = 23) By means of sexual reproduction, a haploid sperm cell from the father fuses with a haploid ovum of the mother - this is called FERTILIZATION or SYNGAMY - the result is the fertilized egg or ZYGOTE - the zygote and all other cells having 2 sets of chromosomes are DIPLOID (2n)

15 As a human develops from a zygote to a sexually mature adult, genes are passed on to all somatic cells of the body by the process of mitosis - the only cells NOT produced by mitosis are the gametes, which develop in the gonads - sexually reproducing organisms must carry out a process that halves the chromosome number in the gametes (compensating for fertilization)

16 MEIOSIS- form of cell division that occurs only in the ovaries or testes - mitosis conserves chromosome number and meiosis reduces chromosome number


18 OTHER SEXUAL LIFE CYCLES There are 3 main types of life cycles: 1. Animals (including humans) - gametes are the only haploid cells - meiosis occurs during production of gametes; no other cell division takes place before fertilization - diploid zygote divides by mitosis, producing a diploid, multicellular organism


20 2. Fungi and algae - gametes fuse to form zygote, and then meiosis occurs before the offspring develop - this produces haploid cells that divide by mitosis to give rise to a haploid multicellular adult organism - the haploid organism produces gametes by mitoisis - only diploid stage is zygote


22 3. Plants and some algae - called ALTERNATION OF GENERATIONS - includes both haploid and diploid multicellular stages - multicellular diploid stage is called the SPOROPHYTE - meiosis produces haploid cells called SPORES

23 - a spore gives rise to a multicellular individual without fusing with another cell - a spore divides mitotically to generate a multicellular haploid stage called the GAMETOPHYTE - the gametophyte makes gametes by mitosis - fertilization produces a diploid zygote, which becomes the next sporophyte


25 CLOSER LOOK AT MEIOSIS Meiosis, like mitosis, is preceded by the replication of chromosomes - this SINGLE replication is followed by two consecutive cell divisions called MEIOSIS I and MEIOSIS II - these divisions result in 4 haploid daughter cells (half as many chromosomes as parent cell)

26 INTERPHASE - chromosomes replicate - for each chromosome, the result is 2 genetically identical sister chromatids attached at their centromeres - centrosomes also replicate

27 PROPHASE I - longer and more complex than prophase in mitosis - chromosomes begin to condense, and homologous chromosomes pair up  called SYNAPSIS - in synapsis, a protein structure attaches the homologous chromosomes tightly together all along their lengths

28 - the pair of homologous chromosomes is known as a TETRAD- a cluster of 4 chromatids - at various places along their length, chromatids are crisscrossed  called CHIASMATA - the chromosomes can trade segments at the chiasmata

29 - centrosomes move away from each other and spindle microtubules form between them - nucleoli and nuclear envelope disappear - spindle microtubules capture the kinetochores and chromosomes begin moving toward metaphase plate - Prophase I takes up about 90% of the time required for meiosis

30 METAPHASE I - chromosomes are arranged on the metaphase plate, STILL IN HOMOLOGOUS PAIRS

31 ANAPHASE I - spindle apparatus guides the movement of the chromosomes toward the poles - sister chromatids remain attached at their centromeres - homologous chromosomes move toward opposite poles of the cell

32 TELOPHASE I AND CYTOKINESIS - each pole of the cell has a haploid chromosome set, but each chromosome still has 2 sister chromatids - cleavage furrows form in animal cells and cell plates appear in plant cells

33 PROPHASE II - a spindle apparatus forms and chromosomes progress toward the metaphase plate

34 METAPHASE II - chromosomes are positioned on metaphase plate (now very similar to mitosis)

35 ANAPHASE II - centromeres of sister chromatids finally separate - sister chromatids of each pair (now individual chromosomes) move toward opposite poles of the cell

36 TELOPHASE II AND CYTOKINESIS - nuclei form at opposite poles of the cell and cytokinesis occurs - there are now 4 HAPLOID DAUGHTER CELLS

37 MITOSIS VS. MEIOSIS MITOSIS - DNA replication occurs during interphase before nuclear division - there is 1 division, including prophase, metaphase, anaphase, and telophase - synapsis does NOT occur - 2 diploid daughter cells are produced that are GENETICALLY IDENTICAL to parent

38 - role in the body: enables multicellular adult to arise from zygote; produces cells for growth and repair

39 MEIOSIS - DNA replication occurs only once, during interphase before meiosis I - there are 2 divisions, EACH including prophase, metaphase, anaphase, and telophase - synapsis does occur; crossing over is associated with synapsis

40 - 4 haploid cells are formed, each having half as many chromsomes as the parent cell - role in body: produces gametes; reduces chromosome number by half and introduces genetic variability


42 The following are responsible for most genetic variation in SEXUALLY REPRODUCING organisms: Independent assortment of chromosomes Crossing over Random fertilization

43 INDEPENDENT ASSORTMENT During metaphase, the orientations of homologous pairs of chromosomes relative to the poles of the cell are random - each gamete represents one outcome of all possible combinations of maternal and paternal chromosomes - the number of combinations possible for gametes formed by meiosis starting with 2 homologous pairs of chromosomes is 4

44 In general, the number of combinations possible when chromosomes assort independently into gametes is 2 n - n is the haploid number of the organism - in humans, n = 23 so 2 23 = about 8 million - the number of possible combinations of maternal and paternal chromosomes in the resulting gametes is about 8 million

45 - each gamete that a human produces contains about one of 8 million possible assortments of chromosomes inherited from the father and mother


47 CROSSING OVER Crossing over produces RECOMBINANT CHROMOSOMES, which combine genes inherited from our 2 parents - crossing over begins very early in prophase I - homologous portions of 2 nonsister chromatids trade places - at metaphase II, chromosomes containing recombinant chromatids can be oriented in 2 alternative ways

48 - the independent assortment of these nonidentical sister chromatids increases the genetic variability in gametes


50 RANDOM FERTILIZATION A human ovum representing one of about 8 million possible chromosome combinations is fertilized by a single sperm cell representing one of 8 million possible combinations - even without crossing over, a zygote is produced with any of 64 trillion combinations

51 EVOLUTION AND VARIATION Darwin recognized the importance of genetic variation in natural selection: - individuals best suited to their environment leave the most offspring, passing on their genes to them - this natural selection results in adaptation, the accumulation of favorable genetic variations

52 - different genetic variations may work better in old environments than new

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