6.3 Meiosis

Question 1

Reduction division

Answer 1

Gametes formed via meiosis form 4 daughter cells due to the nucleus dividing twice
–> each gamete contains half of the chromosome number (haploid)

Question 2

Homologous chromosomes

Answer 2

each nucleus of organism’s cells contain two full sets of genes, a pair of genes for each characteristic. Therefore, each nucleus contains matching sets of chromosomes (homologous chromosomes)

Question 3

Alleles

Answer 3

different versions of the same gene that code for a specific characteristic
–> found in the same locus
–> homologous chromosomes have same genes in same positions= same length and size, centromeres in same position

Question 4

Meiosis I

Answer 4

first division (reduction division ) when pair of homologous chromosomes are separated into two cells
–> each intermediate cell will only contain one full set of genes (haploid)

Question 5

Meiosis II

Answer 5

second division is similar to mitosis & pairs of chromatids present in each daughter cell are separated, forming two more cells
–> 4 haploid cells in total

Question 6

Prophase I (Meiosis I)

Answer 6

Chromosomes condense; nuclear envelope disintegrates; nucleolus disappears and spindle formation begins, as in prophase of mitosis

Difference: homologous chromosomes line up to form bivalents
–> Chromosomes are large molecules of DNA & moving them through cytoplasm to bring them together results in entanglement: crossing over)

Question 7

Bivalent

Answer 7

homologous chromosomes that are in a synapse during meiosis
–> 2 chromosomes and 4 arms/ chromatids (Known as a tetrad)

Question 8

Metaphase 1 (Meiosis I)

Answer 8

same as metaphase except PAIR of chromosomes assemble along metaphase plate (not individual chromosomes)
–> ORIENTATION of each pair is random and independent to any other pair.
–> Individual assortment: Maternal or paternal chromosomes can end up facing either pole (allows many different combinations and results in genetic variation)

Question 9

Anaphase 1 (Meiosis I)

Answer 9

DIFFERENT TO MITOSIS: homologous chromosomes are pulled to opposite poles and chromatids stay joined to each other
–> Sections of entangled DNA on chromatids now break off and rejoin together (sometimes causes an exchange of DNA: Chiasmata)

Question 10

Chiasmata

Answer 10

points at which the chromatids break and rejoin

Question 11

How does genetic variation arise at the anaphase 1 of meiosis I

Answer 11

when exchange occurs, recombinant chromatics are formed with genes being exchanged between chromatids
–> genes being exchanged may be different alleles of the same gene, resulting in combinant of alleles on recombinant chromatids will be different from allele combination on either original chromatic

–> genetic variation arises from this new combination of alleles (sister chromatids are no longer identical)

Question 12

Telophase 1 (Meiosis I) & cytokinesis 1

Answer 12

SAME AS MITOSIS: chromosomes assemble at each pole & nuclear membrane reforms as chromosomes uncoil

Cell undergoes cytokinesis and divides into two (reduction of chromosome number from diploid to haploid is complete

Question 13

Prophase 2 (Meiosis II)

Answer 13

Chromosomes (still consists of 2 chromatids) condense & become visible again
–> nuclear envelope breaks down and spindle formation begins

Question 14

Metaphase 2 (Meiosis II)

Answer 14

DIFFERENT from metaphase 1: individual chromosomes line up on metaphase plate
–> due to crossing over, chromatids are no longer identical (some longer than others)
–> independent assortment occurs again and more genetic variation is produced

Question 15

Anaphase 2 (Meiosis II)

Answer 15

Anaphase 2 results in chromatids of individual chromosomes being pulled to opposite poles after division of centromeres (same as mitosis anaphase)

Question 16

Telophase 2 (Meiosis II) & cytokinesis 2

Answer 16

Chromatids assemble at poles (same as mitosis telophase)
–> chromosomes uncoil and form chromatin again & nuclear envelope reforms and the nucleolus becomes visible
–> Cytokinesis results in 4 daughter cells being formed (haploid due to reduction division & genetically different from each other)