Presentation is loading. Please wait.

Presentation is loading. Please wait.

Chapter 13 Overview: Hereditary Similarity and Variation

Similar presentations

Presentation on theme: "Chapter 13 Overview: Hereditary Similarity and Variation"— Presentation transcript:

1 Chapter 13 Overview: Hereditary Similarity and Variation Living organisms Are distinguished by their ability to reproduce their own kind WARNING: This chapter relies heavily on a knowledge of definitions. Terminology is crucial to understanding meiosis.

2 Heredity Is the transmission of traits from one generation to the next Variation Shows that offspring differ somewhat in appearance from parents and siblings

3 Genetics Is the scientific study of heredity and hereditary variation

4 Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes

5 Inheritance of Genes Genes Are the units of heredity
Are segments of DNA Segments of nucleotides (A, T, C, G) arranged in sequence

6 Each gene in an organism’s DNA
Has a specific locus on a certain chromosome Each chromosome is DNA and proteins May have 100’s or 1000’s of genes We inherit One set of chromosomes from our mother and one set from our father

7 Comparison of Asexual and Sexual Reproduction
In asexual reproduction One parent produces genetically identical offspring by mitosis Figure 13.2 Parent Bud 0.5 mm This gives rise to clones – a group of genetically identical individuals

8 In sexual reproduction
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

9 Fertilization and Meiosis
Concept 13.2: A life cycle Is the generation-to-generation sequence of stages in the reproductive history of an organism

10 Sets of Chromosomes in Human Cells
In humans Each somatic cell has 46 chromosomes, made up of two sets One set of chromosomes comes from each parent Somatic cells – all the cells other than sperm or egg

11 A karyotype Is an ordered, visual representation of the chromosomes in a cell 5 µm Pair of homologous chromosomes Centromere Sister chromatids Figure 13.3

12 Homologous chromosomes
Are the two chromosomes composing a pair Have the same characteristics (length, staining pattern, position of centromere) May also be called autosomes

13 Sex chromosomes Are distinct from each other in their characteristics
Are represented as X and Y Determine the sex of the individual, XX being female, XY being male

14 A diploid cell Has two sets of each of its chromosomes
One set came from mom, one from dad Humans have 46 chromosomes (2n = 46) n = the number of chromosomes in a set

15 In a cell in which DNA synthesis has occurred
Remember . . . In a cell in which DNA synthesis has occurred All the chromosomes are duplicated and thus each consists of two identical sister chromatids Figure 13.4 Key Maternal set of chromosomes (n = 3) Cell is in G2 phase 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

16 Unlike Somatic Cells, Gametes - sperm and egg cells, are haploid cells, containing only one set of chromosomes The haploid number is n = 23 in humans 22 autosomes 1 single sex chromosome

17 Behavior of Chromosome Sets in the Human Life Cycle
At sexual maturity The ovaries and testes produce haploid gametes by meiosis

18 During fertilization The zygote
These gametes fuse, forming a diploid zygote The zygote Develops into an adult organism

19 The human life cycle Key Figure 13.5 Haploid gametes (n = 23)
Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

20 Meiosis – Reduces the Number of Chromosomes
Concept 13.3: Meiosis Takes place in two sets of divisions, meiosis I and meiosis II Results in 4 daughter cells, each genetically unique

21 The Stages of Meiosis An overview of meiosis Figure 13.7 Interphase
Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous separate Haploid cells with replicated chromosomes Sister chromatids Haploid cells with unreplicated chromosomes Meiosis I Meiosis II

22 Meiosis I Reduces the number of chromosomes from diploid to haploid Meiosis II Produces four haploid daughter cells

23 Details of Meiosis Stages
CAUTION! Stages of meiosis I and II use the same names as stages in mitosis There are differences in these stages though!

24 Stages of Meiosis Chromosomes replicated in S phase

25 Stages of Meiosis I Chromosomes begin to condense
Homologous chromosomes loosely pair along their lengths In crossing over, DNA of nonsister chromatids break and rejoin to the other’s DNA In synapsis homologues are held tightly along length Tetrads – group of 4 chromosomes are visible Also, all the happenings of Prophase seen in mitosis occur too

26 Stages of Meiosis I Pairs of homologous chromosomes (in tetrads) move to the middle of the metaphase plate Both chromatids of a homologous chromosome are attached to one kinetochore microtubule

27 Stages of Meiosis I Chromosomes move to opposite poles
Sister chromatids remain attached at centromere

28 Stages of Meiosis I Each cell has a haploid set of chromosomes, but each chromosome still has sister chromatids No chromosome replication takes place before meiosis II

29 Stages of Meiosis II Spindles form
By end of Prophase II, they attach at centromere

30 Stages of Meiosis II Chromosomes moved to the middle of metaphase plate Sister chromatids are not genetically identical

31 Stages of Meiosis II Sister chromatids come apart, move to opposite poles

32 Stages of Meiosis II Meiotic division of 1 cell produces 4 genetically different daughter cells Each daughter cell has a haploid number of chromosomes

33 A Comparison of Mitosis and Meiosis
Meiosis I and II can be distinguished from mitosis because: Meiosis reduces the number of chromosome sets Mitosis conserves the number of chromosome sets And also by: By three events in Meiosis l

34 1) Synapsis and crossing over
Homologous chromosomes physically connect and exchange genetic information

35 2) Tetrads on the metaphase plate
At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates

36 3) Separation of homologues
At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell In anaphase II of meiosis, the sister chromatids separate

37 (before chromosome replication)
A comparison of mitosis and meiosis Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n Daughter cells of mitosis 2n = 6 Text box of Figure 13.9 is excellent!

38 Genetic Variation and Evolution
Concept 13.4: Reshuffling of genetic material in meiosis Produces genetic variation

39 Origins of Genetic Variation Among Offspring
In species that reproduce sexually The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

40 1) Independent Assortment of Chromosomes
Homologous pairs of chromosomes Orient randomly at metaphase I of meiosis

41 In independent assortment
Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs Key Maternal set of chromosomes Paternal set of Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Figure 13.10

42 2) Crossing Over Crossing over
Produces recombinant chromosomes that carry genes derived from two different parents Figure 13.11 Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

43 3) Random Fertilization
The fusion of gametes Will produce a zygote with any of about 64 trillion diploid combinations

44 Evolutionary Significance of Genetic Variation Within Populations
Is the raw material for evolution by natural selection

Download ppt "Chapter 13 Overview: Hereditary Similarity and Variation"

Similar presentations

Ads by Google