Meiosis

Question 1

Fertilization

Answer 1

aka Syngamy Fusion of gametes

Question 2

Gametes

Answer 2

sex cells which are haploid. have half the number of chromosomes of somatic cells.

Question 3

Reduction division

Answer 3

gamete
formation needed to be a process involving
reduction in the number of chromosomes i.e.
meiosis

Question 4

Meiosis and fertilization together constitute a cycle

of reproduction called?

Answer 4

Sexual reproduction

Question 5

Somatic tissues

Answer 5

-All sexual reproduction 
organisms follow alternation 
between diploid and haploid 
chromosome numbers
– After fertilization single cell 
zygote gives rise to all cells in 
adult

Question 6

Germ-Line Tissues

Answer 6

-In animals the cells that will 
undergo meiosis to produce 
gametes are set aside early in 
development
– Both somatic and germ-line 
cells are diploid but germ line 
undergoes meiosis to produce 
haploid gametes

Question 7

What are the 3 unique features of Meiosis?

Answer 7

1. Synapsis
2. Homologous Recombination
3. Reduction Division

Question 8

Synapsis

Answer 8

– Occurs early during first nuclear
division
– Homologous chromosomes pair
along length [side by side]

is the pairing of two chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis.

Question 9

Homologous Recombination

Answer 9

– Genetic exchange occurs between
the homologous chromosomes
while joined
– Called CROSSING OVER

Question 10

Reduction Division

Answer 10

– Chromosomes do not replicate

between the two nuclear divisions.

Question 11

5 Stages of Prophase I?

Answer 11

1. Leptotene
   - Chromosomes condense tightly
2. Zygotene
   -A lattice of protein laid down between
   homologues in synapsis –called synaptonemal complex
3. Pachytene
   -After synapsis , complex keeps
   chromosomes and genes exactly across from each
   other – DNA duplex unwinds and makes links with
   complementary strands of homologue
4. Diplotene
   -Protein lattice complex breaks down –
   intense cell growth occurs – chromosomes decondense
   and become active in transcription
5. Diakinesis
   -Transition into metaphase, transcription
   ceases and chromosomes recondense

Question 12

Synaptonemal complex

Answer 12

A lattice of protein laid down between

homologues in synapsis

Question 13

CROSSING OVER

Answer 13

In synaptonemal complex recombination is facilitated in
pachytene by large proteins called recombination nodules
• DNA is exchanged between non sister and sister chromatids
• In humans average of two or three crossover events occur per
chromosome pair
• Evidence of chiasma (X-shaped structure)shows crossing over
event
• Indicates that two chromatids (one from each homologue)
have exchanged parts
• In diakinesis chiasmata move down the chromosomes

Question 14

Metaphase I

Answer 14

Just like mitosis nuclear envelope disperses and spindle
formed
• Terminal chiasmata hold homologues together and only
allows one side of the centromere to face outwards
• Kinetochore and microtubule can only attach to one
side

Question 15

Anaphase I

Answer 15

Microtubules shorten and break the chiasmata
• Pull centromeres to the poles
• Each pole has a haploid set of chromosomes
consisting of one member of each homologue
• Because of random arrangement of homologous
chromosomes on each plate each pole can receive
either a maternal or paternal homologue from each
chromosome pair
• Genes on different chromosomes assort
independently
• Independent assortment of maternal chromosomes
into gametes.

Question 16

After Meiosis I:

Answer 16

sister chromatids are not identical.

Question 17

The Second Meiotic division.

Answer 17

• After brief interphase with no DNA synthesis
• Prophase II: nuclear envelope breaks down as new
spindle forms
• Metaphase II: spindle fibres bind to both sides of
centromeres
• Anaphase II: Spindle fibres contract - splitting
centromeres and chromatids to opposite poles
• Telophase II: Nuclear envelope forms around four
sets of daughter chromosomes
• Results in four cells containing haploid sets of
chromosomes –no two are alike due to crossing
over.

Question 18

Asexual reproduction

Answer 18

– Individual inherits all chromosomes from one parent
– Individual is genetically identical to parent
– Bacterial cells reproduce by binary fission
– Protists divide asexually unless under stress

Question 19

Budding off and Fragmentation

Answer 19

Multicellular organisms
– May reproduce by budding off localized masses of cells
– Sponges reproduce asexually by fragmentation.

Question 20

Parthenogenesis

Answer 20

Development from an unfertilized egg.
Example: bees
– Fertilized eggs become diploid females
– Unfertilized eggs become haploid males

Question 21

The evolutionary consequences of sex.

Answer 21

Principal Factors in the Evolution of the Eukaryotes
– Reassortment of genetic material occurs during meiosis
– Represents an enormous factor in initiation of genetic
variability
• In humans 23 chromosomes are from each parent
• Each chromosome segregates independently of all
others
• Gamete possibilities equals 223 (over eight million)
• Fertilization squares the number of possibilities (70
trillion)
• Crossing over further adds to the variability

Question 22

Evolutionary Process Is Revolutionary and Conservative

Answer 22

– Revolutionary as the pace is quickened by genetic variability
– Conservative as variation is not always favoured by selection
– Acts to preserve existing combinations of genes

Question 23

Conservative process in asexual organisms

Answer 23

Conservative process greater in asexual organisms that
are not highly mobile
• Live in extremely demanding habitats
• In vertebrates, the evolutionary premium is on
versatility, thus sexual reproduction is predominant

Question 24

What increases pace of evolution?

Answer 24

Pace of evolution increased with greater genetic
diversity.

Genetic diversity is raw material of evolution

Question 25

What is the evolutionary rationale for sexual

reproduction?

Answer 25

• Problems associated with sexual reproduction
• Advantage to species and populations -which benefit
from genetic variability – e.g. vertebrates
• But evolution occurs because of changes at level of the
individual – not at population level therefore sexual
reproduction offers limited or no advantage to
offspring of sexual reproduction
• Genetic recombination could be destructive – not all
changes will favour the individual
• More often it is destructive
• Genetic recombination disrupts advantageous genes
more than it creates new better adapted combinations