Meiosis (3.3) Think back to mitosis! What are the steps?? What happens during each one?

Size: px

Start display at page:

Download "Meiosis (3.3) Think back to mitosis! What are the steps?? What happens during each one?"

Florence Owens
5 years ago
Views:

1 Meiosis (3.3) Think back to mitosis! What are the steps?? What happens during each one? Essen%al Idea: Meiosis produces gene.cally-varied, haploid cells needed for sexual reproduc.on

3.3.1 One diploid nucleus divides by meiosis to produce four haploid nuclei Meiosis is a process that divides one diploid eukaryo1c nucleus to form four haploid

2 3.3.1 One diploid nucleus divides by meiosis to produce four haploid nuclei Meiosis is a process that divides one diploid eukaryo1c nucleus to form four haploid nuclei The original diploid cell is divided twice in this process Meiosis I and Meiosis II Since the chromosome number is halved, Meiosis is called a reduc<on division

3 3.3.U1 One diploid nucleus divides by meiosis to produce four haploid nuclei. Meosis is a reduc<on division of the nucleus to form haploid gametes One diploid (2N) body cells contain a homologous pair of each chromosome (except for sex cells) Four haploid (N) gametes contain one of each chromosome Chromosomes are replicated to form sister chroma1ds First division of the nucleus Second division of the nucleus Edited from: heps://commons.wikimedia.org/wiki/file:diagram_of_meiosis.svg

human diploid cell à 2x23 = 46 chromosomes Chromosomes replicate in s-phase (interphase) to make pairs of sister chroma1ds,

4 3.3.1 One diploid nucleus divides by meiosis to produce four haploid nuclei Soma1c cell nuclei diploid (2n) - contain a homologous pair of each chromosome. - in humans: 23 pairs of chromosomes (n=23). human diploid cell à 2x23 = 46 chromosomes Chromosomes replicate in s-phase (interphase) to make pairs of sister chroma1ds, connected at the centromere. Meiosis has two divisions: Meiosis I separates the homologous pairs (this is the reduc1on division). Meiosis II separates the sister chroma1ds

3.3.3 DNA is replicated before meiosis so that all

of the diploid cell undergoing division contains

5 3.3.3 DNA is replicated before meiosis so that all chromosomes consist of two sister chroma<ds DNA is replicated during the Synthesis (S) phase of Interphase, before Meiosis As a result, the nucleus of the diploid cell undergoing division contains duplicated chromosomes, each with two iden<cal sister chroma<ds

6 Meiosis Now steps of meiosis Next significance!

7 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells - Diploid cell - Homologous chromosomes (bivalents) pair - Crossing over occurs

8 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells - Bivalents line up at the equator - Random orienta<on (the way they face) of the homologous pairs

3.3.11 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells - Spindle

9 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells - Spindle fibers contract This is the reduc<on division the bivalent is split and half the chromosomes move towards each pole.

3.3.11 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells - Chromosomes

10 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells - Chromosomes uncoil, interphase follows with no replica1on - Cell is now haploid. - Two cells are no longer iden1cal - Cytokinesis occurs

3.3.11 Draw diagrams to show the stages of meiosis resul<ng in the forma<on

11 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells - Haploid cell - Chromosomes (aeached chroma1ds) condense

3.3.11 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four

12 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells - Pairs of sister chroma1ds align at the equator Random orienta1on of chroma1ds

3.3.11 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of

13 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells - Centromeres separate and chroma1ds move to opposite poles

14 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells - New nuclei form (haploid) - Cytokinesis begins - Four haploid gamete cells are made.

3.3.11 Draw diagrams to show the stages of

15 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells

16 Draw diagrams to show the stages of meiosis resul<ng in the forma<on of four haploid cells

17 Why does meiosis maeer? Sexual reproduc1on & varia1on

the union of gametes, doubles chromosome number so to prevent chromosome number from doubling each

18 3.3.2 The halving of the chromosome number allows a sexual life cycle with fusion of gametes In asexual reproduc<on, offspring are gene1cally-iden1cal In sexual reproduc<on, there is gene1c diversity Fer<liza<on, the union of gametes, doubles chromosome number so to prevent chromosome number from doubling each genera1on, sex cells must be haploid. Meiosis enables the sexual life cycle of eukaryotes by producing such haploid cells.

chromosomes means there are 2 n possible orienta1ons in metaphase I and II. That is 2 23 in humans or 8,388,068 different combina1ons in gametes!

19 3.3.U7 Crossing over and random orienta1on promotes gene1c varia1on. Prophase I Metaphase I Metaphase II Crossing-over between non-sister chroma1ds results in recombina<on of alleles Random orienta<on of the homologous chromosomes means there are 2 n possible orienta1ons in metaphase I and II. That is 2 23 in humans or 8,388,068 different combina1ons in gametes! Because both crossing-over and random orienta<on occur during meiosis the result is is effec1vely infinite gene<c varia<on in the haploid gamete. n.b. for a new organism to arise sexually meiosis occurs in both parents further increasing the gene.c varia.on

called synapsis - Crossing Over - a sister chroma1d from each of the homologous chromosomes

20 3.3.4 The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensa<on During Prophase I: - homologous chromosomes pair up this process is called synapsis - Crossing Over - a sister chroma1d from each of the homologous chromosomes form a junc1on at random point(s) to exchange genes results in chroma1ds with new allele combina1ons.

3.3.9 Non-disjunc<on can cause Down syndrome and other chromosomal abnormali<es. Studies showing age of parents influences chances of nondisjunc<on. Meiosis is prone to error.

21 3.3.9 Non-disjunc<on can cause Down syndrome and other chromosomal abnormali<es. Studies showing age of parents influences chances of nondisjunc<on. Meiosis is prone to error. In some cases, chromosomes fail to split properly in either Anaphase I or Anaphase II. This is called Non-Disjunc<on The result is the produc1on of a gamete with an extra chromosome (Trisomy) or a missing chromosome (Monosomy) Incidences of Non-Disjunc1on are strongly correlated with maternal age

22 3.3.9 Non-disjunc<on can cause Down syndrome and other chromosomal abnormali<es.

23 3.3.9 Non-disjunc<on can cause Down syndrome and other chromosomal abnormali<es.

24 3.3.9 Non-disjunc<on can cause Down syndrome and other chromosomal abnormali<es.

25 3.3.9 Non-disjunc<on can cause Down syndrome and other chromosomal abnormali<es.

26 Check yourself. Use your diagrams to respond.

27 Which phase of meiosis is shown? Why? A. Interphase B. Prophase I C. Metaphase I D. Metaphase II Reason:

28 Which phase of meiosis is shown? Why? A. Interphase B. Prophase I C. Metaphase I D. Metaphase II Reason: Homologous pairs are aligned (at equator), so must be metaphase. Crossing-over has already taken place, so must be ader prophase I. Homologous pairs have not yet separated, so must be s1ll in meiosis I (metaphase I).

29 Which phase of meiosis is shown? Why? A. Interphase B. Prophase I C. Metaphase I D. Metaphase II Reason:

30 Which phase of meiosis is shown? Why? A. Interphase B. Prophase I C. Metaphase I D. Metaphase II Reason: Homologous pairs have associated. Crossing-over has taken place. Homologous pairs have not aligned at the equator.

31 Which phase of meiosis is shown? Why? A. Interphase B. Prophase I C. Metaphase I D. Metaphase II Reason: 31

32 Which phase of meiosis is shown? Why? A. Interphase B. Prophase I C. Metaphase I D. Metaphase II Reason: Homologous pairs have not yet associated. Replica1on has taken place. Crossing-over has not yet taken place. 32

10.1 Meiosis (AHL) Essen1al idea: Meiosis leads to independent assortment of chromosomes and unique composi1on of alleles in daughter cells.

10.1 Meiosis (AHL) Essen1al idea: Meiosis leads to independent assortment of chromosomes and unique composi1on of alleles in daughter cells. The family portrait shows large amounts of varia1on within a