MEIOSIS Topic 3.3 IB Biology Miss Werba Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes. Topic 3.3 IB Biology Miss Werba

GENETIC MODIFICATION and BIOTECHNOLOGY TOPIC 3 – GENETICS 3.1 GENES 3.2 CHROMOSOMES 3.3 MEIOSIS 3.4 INHERITANCE 3.5 GENETIC MODIFICATION and BIOTECHNOLOGY J WERBA – IB BIOLOGY 2

THINGS TO COVER U.1 U.2 U.3 U.4 U.5 U.6 U.7 U.8 Statement Guidance One diploid nucleus divides by meiosis to produce four haploid nuclei. U.2 The halving of the chromosome number allows a sexual life cycle with fusion of gametes. U.3 DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids. U.4 The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation. The process of chiasmata formation need not be explained. U.5 Orientation of pairs of homologous chromosomes prior to separation is random. U.6 Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number. U.7 Crossing over and random orientation promotes genetic variation. U.8 Fusion of gametes from different parents promotes genetic variation. J WERBA – IB BIOLOGY 3

THINGS TO COVER A.1 A.2 A.3 S.1 Statement Guidance NOS 1.8 Non-disjunction can cause Down syndrome and other chromosome abnormalities. A.2 Studies showing age of parents influences chances of non-disjunction. A.3 Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks. S.1 Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells. Drawings of the stages of meiosis do not need to include chiasmata. NOS 1.8 Making careful observations. J WERBA – IB BIOLOGY 4

MEIOSIS U.1 Meiosis is a reduction division of a diploid nucleus to form four haploid nuclei. J WERBA – IB BIOLOGY 5

Making careful observations DISCOVERY OF MEIOSIS NOS 1.8 Making careful observations Meiosis was discovered by microscope examination of dividing germ-line cells. J WERBA – IB BIOLOGY 6

DISCOVERY OF MEIOSIS NOS 1.8 First observed in 1876 by Oscar Hertwig, as he studied sea urchin eggs under a microscope. Eduord van Beneden observed the behaviour of chromosomes during meiosis in the eggs of parasitic worms. In 1890, August Weismann observed the two phases needed to complete the division of eggs. The term meiosis was first used in 1905. J WERBA – IB BIOLOGY 7

MEIOSIS U.2 The halving of the chromosome number prevents doubling of the DNA with fusion of gametes during fertilisation. J WERBA – IB BIOLOGY 8

MEIOSIS U.2 Occurs in the diploid germ cells in the gonads (testes & ovaries) Meiosis involves 2 divisions - Meiosis I and Meiosis II J WERBA – IB BIOLOGY 9

MEIOSIS Produces four gametes (sex cells – sperm or ova) Gametes are haploid and not identical to original cell J WERBA – IB BIOLOGY 10

non-sister chromatids MEIOSIS U.3 DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids. sister chromatids non-sister chromatids J WERBA – IB BIOLOGY 11

MEIOSIS I J WERBA – IB BIOLOGY 12

MEIOSIS I Interphase DNA replication and preparation of the cell U.4 J WERBA – IB BIOLOGY 13

MEIOSIS I U.4 U.5 U.6 Prophase I Homologous chromosomes pair up (now called a bivalent). Crossing over may occur: Non-sister chromatids may cross over at points (called chiasmata) and exchange genetic material Spindle microtubule forms The nuclear envelope breaks down. J WERBA – IB BIOLOGY 14

MEIOSIS I Metaphase I Homologous pairs line up randomly at equator. Maternal and paternal chromosomes of each pair line up independently of other pairs = independent assortment. This greatly increases genetic variety in the gametes. J WERBA – IB BIOLOGY 15

MEIOSIS I Anaphase I The microtubules contract. Homologous chromosomes separate and move towards opposite poles. The chromosome number is halved from diploid to haploid. Chromosomal non- disjunction occurs if they fail to separate J WERBA – IB BIOLOGY 16

MEIOSIS I Telophase I + Cytokinesis Spindles break down. U.4 U.5 U.6 Telophase I + Cytokinesis Spindles break down. New nuclear envelope forms around each new nucleus. Two new haploid cells form. Each chromosome still has 2 sister chromatids at this point. J WERBA – IB BIOLOGY 17

CROSSING OVER & RECOMBINATION U.4 U.7 Occurs in Prophase I Sections of non-sister chromatids may touch This crossing-over point is called a chiasma (pl. chiasmata). Sections of the chromosomes may be swapped between the non-sister chromatids. This produces recombinant chromosomes (ie. unlike either parent chromosome) Increases variation J WERBA – IB BIOLOGY 18

CHROMOSOMAL NON-DISJUNCTION Occurs in Anaphase I Homologous chromosomes fail to separate properly. This is a form of chromosomal mutation. Extra or fewer chromsomes are drawn to each pole and packaged into the new cells. Results in trisomy (3 copies) or monosomy (1 copy) eg. Down Syndrome = trisomy 21 The chance of non-disjunction increases with maternal age. Ref: Biology Key Ideas J WERBA – IB BIOLOGY 19

MEIOSIS II J WERBA – IB BIOLOGY 20

MEIOSIS II U.4 U.5 U.6 Prophase II New spindle microtubules form in each of the two cells. The spindles form at right angles to the original division. The nuclear envelope breaks down. J WERBA – IB BIOLOGY 21

MEIOSIS II Metaphase II Individual chromosomes line up at equator. Spindle fibres attach to each chromatid at the centromere. J WERBA – IB BIOLOGY 22

MEIOSIS II Anaphase II The microtubules contract. Sister chromatids separate and move towards opposite poles. J WERBA – IB BIOLOGY 23

MEIOSIS II Telophase II + Cytokinesis Spindles break down. U.4 U.5 U.6 A.1 Telophase II + Cytokinesis Spindles break down. New nuclear envelopes form around each new nucleus. The chromosomes uncoil. Four haploid cells form. Each cell formed is genetically unique. J WERBA – IB BIOLOGY 24

https://youtu.be/eaf4j19_3Zg?t=15s

MEIOSIS PROMOTES GENETIC VARIATION U.5 U.6 U.7 U.8 Each chromosome of the homologous pair carries different alleles. Homologous pairs line up and separate independently. Crossing over may occur during prophase I leading to entirely new combinations of alleles. There is fusion of gametes from different parents during fertilisation. J WERBA – IB BIOLOGY 26

DOWN SYNDROME A.1 A.2 A.3 Down syndrome is a condition resulting from a non- disjunction in the chromosome 21 homologous pair. It is also called Trisomy 21. Karyotyping can be used to detect Down syndrome in foetuses. The process utilises foetal DNA obtained using chorionic villus sampling (CVS) or amniocentesis. New techniques use a maternal blood test to access foetal DNA. J WERBA – IB BIOLOGY 27

DOWN SYNDROME Why is prenatal testing needed? All women are at risk of having a baby with Down syndrome, although the risk increases with the age of the woman. If the mother is over 35, prenatal testing is strongly encouraged. J WERBA – IB BIOLOGY 28

DOWN SYNDROME MALE KARYOGRAM FEMALE KARYOGRAM A.1 A.2 A.3 J WERBA – IB BIOLOGY 29

PRE-NATAL TESTING CHORIONIC VILLUS SAMPLING 11-12 weeks of pregnancy Sample of the chorionic villi (from the placenta) is taken A karyogram is made & analysed Procedural risk is ~1% J WERBA – IB BIOLOGY 30

PRE-NATAL TESTING AMNIOCENTESIS 16 weeks of pregnancy Sample of amniotic fluid (contains foetal cells) is taken A karyogram is made & analysed Procedural risk is ~0.5% J WERBA – IB BIOLOGY 31

PRE-NATAL TESTING NON-INVASIVE PRE-NATAL TESTING (NPT) Any stage of pregnancy (usually before end of 2nd trimester) Examines DNA from the unborn baby circulating in the mother’s bloodstream A karyogram is made & analysed Procedural risk is 0% J WERBA – IB BIOLOGY 32

MEIOSIS Q1. If the haploid number of a cell is represented by the letter X, what would be the number of chromosomes in the cells at the end of the first division of meiosis be? 4X X 2X 0.5X J WERBA – IB BIOLOGY 33

MEIOSIS Q2. If the haploid number of a species is 14, how many chromatids will there be in metaphase I in a dividing diploid cell? 56 7 28 14 J WERBA – IB BIOLOGY 34

MEIOSIS Q3. What is chorionic villus sampling? Sampling cells from the foetal digestive system Sampling cells from the placenta Sampling cells from the amniotic fluid Sampling cells from the umbilical cord J WERBA – IB BIOLOGY 35

MEIOSIS Q4. Outline how Down syndrome occurs due to meiosis. (2 marks) J WERBA – IB BIOLOGY 36

MEIOSIS A1. B A2. A A3. B A4. occurs due to non-disjunction; (homologous) chromosome/pairs/sister chromatids fail to separate; some gametes have an extra chromosome; J WERBA – IB BIOLOGY 37