Meiosis: Formation of Germ Cells, Universidad Europea

UNIT 5

Meiosis: Formation of Germ Cells

1. Sexual reproduction and meiosis
2. Features of meiosis
3. Phases of meiosis
4. Comparison mitosis vs meiosis
5. Genetic variation
6. Formation of gametes
7. Fertilization

QUESTION EVERYTHING

ue Universidad Europea¿What is meiosis?

ue Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four haploid cells.

Homologous pair X X XX Haploid (n) 2 chromosomes

XX Diploid (2n) 4 chromosomes X It is a sequence of two nuclear divisions: · The first division is reductional (2n -> n) (homologous chromosomes are distributed). · The second division is similar to mitosis (sister chromatids are distributed).Remember ... . Homologous chromosomes: Present in diploid (2n) organisms.

Diploid (2N) Haploid (N) One chromosome of each homologous pair comes from the mother (called a maternal chromosome) and one comes from the father (paternal chromosome).

Homologous chromosomes are similar but not identical. Each carries the same genes in the same order, but the versions of each gene may not be the same.

D Homologous pair (chromosome 1)

Homologous pair (chromosome 2) Paternal copy (from father) Maternal copy (from mother)3. CHROMOSOMES Other important concepts: Locus (plural loci): A fixed position on a chromosome, such as the position of a gene or genetic marker

19p13.2 19p13.12 19p12 19q12 19q13.13 19q13.31 19q13.33 19q13.42 Alleles: Different variants of a gene

Homologous chromosomes Allele for attached earlobes (e)

Allele for free earlobes (E)

Gene for earlobesChromosomes and alleles In a pair of homologous chromosomes, one is inherited from the male parent, and the other from the female parent.

Paternal homologue

Maternal homologue

KT 1 2 3 4 5 r r 6 7 8 9 10 11 12

P P At each genetic locus, an individual has two alleles, one on each homologous chromosome.

13 14 15 16 17 18

AA = Homozygous - dominant

A A

19 20 21 22 X/Y

bb = Homozygous recessive

b b

- Three gene pairs at three different loci

Cc = Heterozygous-> C C

4 A genetic locus is the location of a particular gene on a chromosome.ue Types of cells in a sexually reproducing organism Somatic cells: These are all the cells that make up the body except for the reproductive cells. E.g .: skin, liver, eyebal. They are diploid (2n) and divide through mitosis > ensure genetic consistency in all body cells.

Germ cells: These are specialized cells involved in reproduction, giving rise to gametes (sperm and egg cells). They produce haploid gametes through meiosis -> create genetic variation.

n 1 1 1 1

C . . L 1 11 12 13 15 - - 5 x . 1 . € 1Sexual reproduction and meiosis Gametes: Only one set of chromosomes > haploid cells (n) During sexual reproduction: the haploid sperm and egg unite to form a single diploid cell and the number of chromosomes is restored in the offspring

Haploid sperm

Paternal homologue

Fertilization - Maternal homologue

Diploid zygote

Haploid egg

The zygote increases their amount of cells by mitotic divisions

Gametes Male (n) Female (n)

Meiosis

Fertilization

HAPLOID (n)

DIPLOID (2n)

Zygote (2n)

Mature organism (2n)

In the diplontic life cycle, the organism is diploid and the gametes are the only haploid stage.Sexual reproduction and meiosis

ue Meiosis produces four gamete cells that are genetically different from each other and from the mother cell (Process of recombination- gene exchange)

Meiosis consists of two divisions with one round of DNA replication: the parent cell undergoes two rounds of cell division (Meiosis I and Meiosis II) with no replication of genetic material between them.

In meiosis I, the homologue chromosomes divide, reducing the chromosome number to the haploid state (n, reductive division).

Meiosis II (second division without prior replication): similar to mitotic división . Sister chromatids become separated.

Each haploid daughter cell (n) " Contains one single set of chromosomes Has a new combination of genes Is different from the mother cell and from the other daughter cells

Increased diversity of the genetic pool (species level)

10Sexual reproduction and meiosis

MITOSIS VS MEIOSIS

ue MITOSIS VS

Before Division Homologous pair of chromosomes one diploid parent cell

Interphase - chromosomes replicate

Homologous pair of replicated chromosomes

5 Meiosis I - first cell division

Homologous pair of replicated chromosomes separates

X Meiosis II - second cell division

Sister chromatids separate

1 four haploid daughter cells

Attack of the clones! @Amoeba Sisters

MEIOSIS 1(A)

MEIOSIS (B)

MITOSIS

paternal homolog

MEIOTICS PHASE

maternal homolog

Į DNA REPLICATION

DNA REPLICATION

Meiosis: S phase + two rounds of chromosome segregation

>

Meiosis I (genetic recombination) Homologues pair up and are segregated into daughter nuclei.

Meiosis II: sister chromatids are segregated.

MEIOSISI

SEGREGATION OF HOMOLOGS AT ANAPHASE I

DUPLICATED CHROMOSOMES LINE UP INDIVIDUALLY ON THE SPINDLE

SEGREGATION OF SISTER CHROMATIDS AT ANAPHASE II

MEIOSIS I

haploid daughter cells

diplold daughter cells

Mitosis: Sister chromatids are segregated. Two genetically identical diploid daughter cells

SEGREGATION OF SISTER CHROMATIDS AT ANAPHASE

15 PAIRING AND CROSSING OVER OF DUPLICATED HOMOLOGS

HOMOLOG PAIRS LINE UP ON THE SPINDLE3. Phases of Meiosis

Meiosis I (1st meiotic division)

• Prophase I:
• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I

Meiosis II (2nd meiotic division)

• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

18Meiosis I (1st meiotic division)

Prophase I

Metaphase I

Anaphase I

Telophase I & cytokinesis

Centrosome Centrioles Spindle Chiasmata Nuclear envelope (Fragment)

Homologous chromosomes

The chromosomes condense, and the nuclear envelope breaks down. Crossing-over occurs.

Pairs of homologous chromosomes move to the equator of the cell.

Centromere (with kinetochore)

Metaphase plate

Sister chromatids

Microtubule

Homologous chromosomes move to the opposite poles of the cell.

Cleavage furrow

Chromosomes gather at the poles of the cells. The cytoplasm divides.

183. Phases of Meiosis: Prohpase I

ue Prophase I: (5 phases: leptotene, zygotene, pachytene, diplotene, diakinesis) " Homologous chromosomes become closely associated thanks to the synaptonemic complex, and exchange segments by crossing over. " The pairing chromosomes stay joined forming a structure called bivalents or tetrads (4 chromatids together).

LEPTOTENE

ZYGOTENE

DIPLOTENE

DIAKINESIS

Nuclear envelope

Chiasma visible

Nuclear envelope fragmenting

replicated paternal chromosome

replicated maternal chromosome

centromere

Synaptonemal complex forming

sister chromatids

Duplicated chromosomes start to condense

Synapsis begins

Synapsis complete; crossing over occurs

Synaptonemal complex disappearing; chiasma visible

Bivalent ready for metaphase (A)

bivalent (B)

chiasma @ 2012 Pearson Education, Inc.

20 Bivalent forming

PROPHASE I PACHYTENE2. Meiosis: Features Prophase I. Recombination: Exhange of genetic material Genetic recombination (or crossing over) allows the homologues to exchange chromosomal material. " Sites of crossing over: chiasmata.

During prophase I, homologous chromosomes, each with a pair of sister chromatids, line up to form a tetrad.

Sister chromatids

Homologous chromosomes

Chiasma

Adjacent chromatids of different homologs break and rejoin. Because there is still sister chromatid cohesion, a chiasma forms.

The chiasma is resolved.

Recombinant chromatids contain genetic material from different homologs.

Recombinant chromatids

Homologous chromosomes

Chiasmata

Centromeres

Homologous chromosomes

Chiasmata

Centromeres

11.17 Chiasmata: Evidence of Genetic Exchange between Chromatids This micrograph shows a pair of homologous chromo- somes, each with two chromatids, during prophase I of meiosis in a salamander. Two chiasmata are visible.

Each chromosome arm usually has one or few crossovers per meiosis. Threy allow for new allelic recombination in offspring >Increases genetic variation

Mitochondrial DNA does not undergo recombination

133. Phases of Meiosis

PROPHASE I: Crossing Over and Recombination

Leptotene Zygotene Pachytene Diplotene Diakinesis Crossing over and recombination

Two of the gametes are parental types form a bivalent

V A C A C bivalent

a C a C a C homologous chromosomes undergo crossing-over

A C

Two of the gametes are recombinant types

a C a C homologous chromosomes contain heterogeneous regions

A C A C C a C a C 50% parental 50% recombinant

A A Cue 3. Phases of Meiosis

Metaphase I

. The bivalent chromosomes are located on the equatorial plate. · Alignment of homologous chromosomes in the center of the mitotic spindle. . The tetrads formed by homologous chromosomes are observed.

Metaphase plate

meiotic metaphase |

chi asma fused kinetochores of sister chromatids function as one

Meiotic spindle ADMIS AE CISTED3. Phases of Meiosis

Anaphase I

Reduction of genetic material ! The homologous chromosomes are separated (MEIOSIS I) instead of the sister chromatids (MITOSIS)

▪ Homologous chromosomes are pulled apart to the opposite poles of the spindle Each chromosome carries fragments intercrossed during the recombination. " Each pole gets n chromosomes with two chromatids > Haploid cells (n)

Chromosome movement

meiotic ana phase |

Kinetochore fibres3. Phases of Meiosis

TELOPHASE I

Chromosomes are now at the opposite poles of the cell " Nuclear membrane re-appears. Note that sister chromatids are no longer identical (due to crossing over)3. Phases of Meiosis

(a) Leptotene (b) Zygotene (c) Pachytene (d) Diplotene (e) Diakinesis (f) Metaphase I (g) Early anaphase | (h) Later anaphase I (i) Telophase I (j) Interphase (k) Prophase II (I) Metaphase II (m) Anaphase II (n) Telophase II (o) The tetrad (p) Young pollen grains3. Phases of Meiosis

MEIOSIS II

ue Daughter cells (n, haploid) undergo another interphase without S-phase (no further replication of DNA)

Very similar to a mitosis but with half the chromosomes: The sister chromatids will separate during the anaphase II

M Go

Prophase Metaphase Anaphase Telophase

Growth

G2 Preparation for Mitosis

Growth

Preparation for DNA Synthesis

G1

Interphase

DNA Replication

S

Prophase II Metaphase II Anaphase II Telophase II