5. Chromosomes and Genetic Diversity

Page 1:

  • DNA (Deoxyribonucleic Acid)

  • Base Pairs

  • Chromosome

  • Telomere

  • p arm

  • Centromere

  • q arm

  • Chromatid

Page 2:

  • Key Knowledge From Chromosomes to genomes

  • Homologous chromosomes and autosomes

  • Variability of chromosomes in different organisms

  • Karyotyping for identifying chromosome abnormalities

  • Meiosis and genetic diversity

Page 3:

  • Prokaryotes vs Eukaryotes

  • DNA packaging in chromosomes

  • Circular chromosomes in prokaryotes

  • Plasmids in prokaryotes

Page 4:

  • DNA packaging in chromosomes

  • Identical copies of DNA in every cell

  • Chromosomes tightly wound around histones and proteins

  • Centromere divides chromosome into two sections

Page 5:

  • Chromosomes in Humans

  • 22 pairs of homologous autosomes

  • 1 pair of sex chromosomes

  • 46 chromosomes in total

  • Same gene located on the same loci of each part of the pair

Page 6:

  • Sex Chromosomes

  • Mammals: female = XX, male = XY

  • Male determines the sex of the offspring

  • X chromosome carries more genes than Y chromosome

Page 7:

  • Structure of Chromosomes

  • Centromere divides chromosome into two sections

  • Arms of the chromosome: p arm and q arm

Page 8:

  • Chromosome size

  • Range in size from 50 million - 300 million base pairs

  • Different number of genes on each chromosome

  • Genes separated by spacer DNA regions

Page 9:

  • Homologous chromosomes and genetic loci

  • Inherited from male and female parents

  • Genetic locus is the location of a gene on a chromosome

  • Homozygous and heterozygous alleles

Page 10:

  • Structure of chromosomes

  • Centromere position varies in different chromosomes

  • Descriptive names given based on centromere position

Page 11:

  • Centromere, chromatids, and chromosomes

Page 12:

  • Diploid numbers of chromosomes in various species

  • Different diploid numbers in animals, plants, and fungi

Page 13:

  • Chromosome numbers of Australian plants

  • Diploid numbers provide evidence of evolutionary relationships

Page 14:

  • Key Knowledge From Chromosomes to Genomes

  • Homologous chromosomes and autosomes

  • Variability of chromosomes in different organisms

  • Karyotyping for identifying chromosome abnormalities

  • Meiosis and genetic diversity

Page 15:

  • Meiosis process and genetic diversity

Page 16:

  • Differences between asexual and sexual reproduction

  • Number of parents or parental contributions

Page 17:

  • Sexual reproduction and genetic contributions from two parents

  • Gametes and gonads

  • Advantage of sexual reproduction in generating genetic diversity

Page 18:

  • Chromosome numbers before and after replication and division

Page 19:

  • Meiosis video

Page 20:

  • Stages of meiosis: Interphase, Prophase I, Metaphase I, Anaphase I, Telophase I, Cytokinesis, Prophase II, Metaphase II, Anaphase II, Telophase II, Cytokinesis

Page 21:

  • Meiosis process and production of gamete cells (sperm and egg)

Page 22

  • Homologous Chromosomes

    • One paternal and one maternal chromosome pair inside a cell during meiosis

  • Crossing Over

    • Process where homologous chromosomes pair up and exchange genetic material to form new chromosomes

  • Haploid Cells

    • Four cells as a result of meiosis, genetically different from each other and the parent cells

Page 23

  • Quick Action - Meiosis Draw

    • Make a quick drawing of homologous chromosome pair crossing over

    • Tells about the genetic makeup of the four gametes (sex cells - sperm or egg)

Page 24

  • Crossing Over

    • Significance: produces chromosomes with new combinations of genetic information

    • Occurs during the first division of Meiosis I

    • Point where crossing over occurs is called Chiasma (Chiasmata plural)

    • New combination of genes referred to as recombinant chromosomes

Page 25

  • Independent Assortment

    • When crossing over is complete, pairs line up along the equatorial plate of the cell in Metaphase I

    • Maternal and paternal chromosomes do not line up on the same side

    • Process called independent assortment

    • Formula 2ⁿ can be used to calculate the number of possible chromosome combinations due to independent assortment

    • n represents the organism's haploid number (23 in humans)

Page 26

  • Meiosis I - Prophase l

    • First phase of Meiosis l

    • Chromosomes become visible under a microscope

    • Duplication of homologous chromosome pair and cross-over occurs

    • Nuclear envelope disappears

    • Spindles enter nucleus

Page 27

  • Meiosis I - Metaphase l

    • Crossover is complete

    • Chromosomes move toward the center and line up

    • Spindle fibers attach to the centromere of each chromosome

Page 28

  • Meiosis I - Anaphase I

    • Cell starts to lengthen

    • Two of each chromosome pairs separate and are pulled by the spindle fibers toward opposite poles

    • In meiosis, the chromatids remain together

    • Contrast with mitosis, where sister chromatids separate

Page 29

  • Meiosis I - Telophase I

    • Chromosomes decondense

    • A cleavage furrow appears

    • By the end of the stage, the parent cell has divided into two daughter cells

    • Separation of cytoplasm called cytokinesis

    • 2 new daughter cells form (Animal Cell: Contractile ring causes a cleavage furrow to form, Plant Cell: Cell plate forms new cell wall between the nuclei)

Page 30

  • Interkinesis in Meiosis

    • Period of rest called interkinesis

    • No replication of DNA occurs during this phase

    • Daughter cells

Page 31

  • Meiosis II - Prophase II

    • First step in Meiosis II

    • Begins with two daughter cells from Meiosis I

    • Chromosomes are condensed

    • Nuclear envelope begins to break down

    • Centrosomes have replicated and are moving toward the poles

    • Crossing over does not occur

Page 32

  • Meiosis II - Metaphase II

    • Second stage of Meiosis II

    • Replicated chromosomes alone at the equatorial plate

    • Spindle fibers attach on either side of the chromosomes' centromere

Page 33

  • Meiosis II - Anaphase II

    • Third step of Meiosis II

    • Very similar to mitosis anaphase

    • Two sister chromatids of each chromosome are pulled apart by the spindle fibers

    • Spindle fibers shorten and separate the sister chromatids to opposite ends

    • Cell elongates so that the poles are farther apart

Page 34

  • Meiosis II - Telophase II

    • Chromosomes reach opposite poles

    • Chromosomes decondense

    • Nuclear envelopes reform

    • Four Haploid cells form

Page 35

  • Meiosis - Cytokinesis

    • Occurs after meiosis II

    • Cytoplasm divides by forming a cleavage furrow

    • Four haploid cells form

    • Meiosis is complete with four daughter cells (haploid) each different from each other and different from the parent cell

Page 36

  • Summary of Meiosis

    • Form of cell division that results in half the number of chromosomes in gametes or sex cells (sperm and ova)

    • Maintains the same number of chromosomes from generation to generation

    • Results in an assortment of genetic material passed on to offspring

Page 37

  • Comparison of Mitosis and Meiosis

    • Mitosis and Meiosis

    • Mitosis: Happens in body cells, a type of cell division

    • Meiosis: Happens in sex organs (testes and ovaries)

Page 38

  • Meiosis I

  • Meiosis II

  • Spermatocyte (diploid)

  • Spermatids

  • Sperm cells (haploid)

  • Oocyte

  • Egg cell (diploid)

  • Polar bodies (haploid)

  • Gamete formation in the male

  • Gamete formation in the female

Page 39

  • Meiosis: source of variation

  • Biological significance of meiosis: produces genetic variability among the offspring produced

  • Variation is produced by:

    • Crossing-over between homologous chromosomes (Prophase I)

    • Independent assortment (separation of homologous chromosomes during meta/anaphase I)

    • Random segregation of non-matching chromosomes

Page 40

  • Unit 2, AOS 1: Key Knowledge

  • From Chromosomes to genomes

  • Nature of a pair of homologous chromosomes carrying the same gene loci and the distinction between autosomes and sex chromosomes

  • Variability of chromosomes in terms of size and number in different organisms

  • Karyotyping as a visual representation that can be used to identify chromosome abnormalities

  • Production of haploid gametes from diploid cells by meiosis, including the significance of crossing over of chromatids and independent assortment for genetic diversity

Page 41

  • Are all chromosomes the same size?

  • Members of each pair are the same size and shape, and they have the same banding patterns

  • Each pair of chromosomes can be a different size, holding a different number of genes

  • Seen in a karyotype

Page 42

  • Human Karyotype

  • Karyotype is the full complement of chromosomes in a particular species (map of chromosomes)

  • Chromosomes differ in size, banding pattern, and location of centromere

  • Chromosomes assigned numbers according to size, except for sex chromosomes

Page 43

  • Chromosomal abnormalities

  • Chromosome change resulting in a syndrome

  • Approximate incidence rate

  • Addition: whole chromosome extra number- 21 (47, +21) Down syndrome 1/700 live births

Page 44

  • Variations in chromosomes

  • Monoploidy: Cell or organism with a functional genome consisting of one copy of each chromosome (1n)

  • Examples: male bees, ants, and wasps

Page 45

  • Variations in chromosomes

  • Polyploidy: Cell or organism with a genome comprising 3 or more copies of each chromosome (3n, 4n, 5n, 6n, etc.)

  • Common in flowering plants, ferns, and green algae

  • Many varieties of fruit are generated polyploids, e.g. seedless grape and strawberries

Page 46

  • Variations in chromosomes

  • Aneuploidy: Condition in which there is an addition or loss of one chromosome from a cell (2n+1 or 2n-1)

  • Result of non-disjunction

Page 47

  • Non-disjunction

  • During meiosis, homologous pairs of chromosomes separate with the help of spindle fibers during anaphase II (disjunction)

  • Non-disjunction: Failure of homologous pairs in meiosis to separate and move to opposite poles

  • Results in the formation of 2 types of gametes - one has 2 copies of a particular chromosome and the other has none

  • Can occur with both autosomes and sex chromosomes

Page 48

  • Non-disjunction

  • Parent cell

  • MEIOSIS I

  • Homologous chromosomes fail to segregate

  • Gametes

  • MEIOSIS II

  • Gametes

Page 49

  • Analyzing karyotypes - Aneuploidy

  • Karyotypes analyzed to find mistakes in chromosome numbers or abnormalities of single chromosomes resulting in congenital disorders

  • Trisomy: Somatic cells contain 3 copies of a particular chromosome

  • Monosomy: Somatic cells contain one copy of a particular chromosome

  • May have a huge effect on the development of a baby, sometimes leading to death

  • Some types of aneuploidy survive in humans, e.g. Down Syndrome

Page 50

  • Analyzing karyotypes - Aneuploidy

  • KB First division nondisjunction

  • Normal disjunction

  • Haploid

  • Trisomic

  • Monosomic

  • Monosomic gamete

Page 51

  • Possible causes of chromosomal abnormalities

    • Duplication, deletion, or translocation of a segment in gametes

    • Offspring can have chromosomes that look different from parents

Page 52

  • Down syndrome karyotype

  • Down Syndrome caused by extra copy of chromosome 21

Page 53

  • Possible cause of Down syndrome

    • Chromosomal non-disjunction during Anaphase I of Meiosis

Page 54

  • Klinefelter's syndrome

    • Also known as 47, XXY

    • Symptoms: infertility, small testicles

    • Other names: XXY syndrome

    • Cause: Two or more X chromosomes in males

Page 55

  • Klinefelter Syndrome karyotype

  • KARYOTYPE: XXY

Page 56

  • Turner syndrome

    • Only affects females

    • Missing or partially missing X chromosome

    • Can cause medical and developmental problems

Page 57

  • Turner syndrome karyotype

  • KARYOTYPE: XX

Page 58

  • Review Questions

    • Heinemann key Questions: 1-9 page 261 Biozone

    • The nature of chromosomes

    • Species have different chromosome numbers

    • Karyotyping

    • Meiosis

    • Meiosis and Variation

  • Heinemann Activity Book:

    • Worksheet 24