REVISION: MEIOSIS Topic 4.2 IB Biology Miss Werba

HAPLOID & DIPLOID Haploid: One set of chromosomes In humans, n = 23 4.2.1 HAPLOID & DIPLOID Haploid: One set of chromosomes In humans, n = 23 In humans, gametes (sperm & ova) are haploid Diploid: Two sets of chromosomes In humans, 2n = 46 In humans, all body cells (other than gametes) are diploid J WERBA – IB BIOLOGY 2

HAPLOID & DIPLOID Diploid: 4.2.2 HAPLOID & DIPLOID Diploid: each pair of chromosomes have the same genes they are arranged in the same sequence (loci) they don’t have the same alleles of all of the genes They are therefore considered to be homologous, not identical. The chromosomes are said to exist in homologous pairs. J WERBA – IB BIOLOGY 3

4.2.3 PROCESS OF MEIOSIS Meiosis =cell division to halve the chromosome number of the cell A reductive division Must occur in organisms that reproduce sexually to prevent doubling of the DNA during fertilisation Occurs in germ cells in the gonads - diploid Production of gametes (sex cells) Produces four gametes from every germ cell Gametes are haploid and not identical to original cell Involves 2 divisions - Meiosis I and Meiosis II J WERBA – IB BIOLOGY 4

MEIOSIS I Homologous chromosomes pair up. They are called a bivalent. 3.5.3 MEIOSIS I Homologous chromosomes pair up. They are called a bivalent. Non-sister chromatids may cross over at points called chiasmata. They may exchange genetic material – crossing over. Homologous pairs line up at equator. Maternal and paternal chromosomes of each pair line up independently of other pairs – independent assortment. Homologous chromosomes separate and move towards opposite poles. Two new haploid cells form. J WERBA – IB BIOLOGY 5

3.5.3 MEIOSIS I Ref: Advanced Biology, Kent J WERBA – IB BIOLOGY 6

MEIOSIS II New spindle apparatus forms. 3.5.3 MEIOSIS II New spindle apparatus forms. Chromosomes line up at the equator in a single line. Centromeres divide and sister chromatids move towards opposite poles. Each cells divides, resulting in a total of four haploid cells. Each cell formed is genetically unique due to crossing over and independent assortment. J WERBA – IB BIOLOGY 7

3.5.3 MEIOSIS II J WERBA – IB BIOLOGY Ref: Advanced Biology, Kent 8

CROSSING OVER & RECOMBINATION 4.2.3 CROSSING OVER & RECOMBINATION Occurs in Prophase I Occurs when the chromosomes (bivalent) line up in pairs Sections of non-sister chromatids may touch (cross over). This point is called a chiasma (pl. chiasmata). Sections of the chromosomes may be swapped between the non-sister chromatids. This produces recombinant chromosomes (ie. chromosomes that are unlike either parent chromosome) Increases variation J WERBA – IB BIOLOGY 9

CHROMOSOMAL NON-DISJUNCTION 4.2.4 CHROMOSOMAL NON-DISJUNCTION Occurs in Anaphase I Occurs when homologous chromosomes fail to separate properly. They are a form of chromosome 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 involves trisomy 21 The chance of non-disjunction increases with maternal age. Ref: Biology Key Ideas J WERBA – IB BIOLOGY 10

4.2.5 KARYOTYPING Karyotyping is arranging the chromosomes in pairs according to their structure. Chromosomes are easily identifiable and are arranged depending upon: their length the position of their centromere Karyotyping can be used to detect chromosome disorders in foetuses. DNA is obtained using Chorionic villus sampling or amniocentesis J WERBA – IB BIOLOGY 11

KARYOTYPING MALE KARYOTYPE FEMALE KARYOTYPE 4.2.5 J WERBA – IB BIOLOGY 12

CHORIONIC VILLUS SAMPLING 4.2.6 CHORIONIC VILLUS SAMPLING 11-12 weeks of pregnancy Sample of the chorionic villi (formed from the outer cells of the zygote) is taken A karyotype is made & analysed Procedural risk is ~1% J WERBA – IB BIOLOGY 13

AMNIOCENTESIS 16 weeks of pregnancy 4.2.6 AMNIOCENTESIS 16 weeks of pregnancy Sample of amniotic fluid (contains foetal cells) is taken A karyotype is made & analysed Procedural risk is ~0.5% J WERBA – IB BIOLOGY 14