Meiosis Flashcards
Fertilization
aka Syngamy Fusion of gametes
Gametes
sex cells which are haploid. have half the number of chromosomes of somatic cells.
Reduction division
gamete
formation needed to be a process involving
reduction in the number of chromosomes i.e.
meiosis
Meiosis and fertilization together constitute a cycle
of reproduction called?
Sexual reproduction
Somatic tissues
-All sexual reproduction organisms follow alternation between diploid and haploid chromosome numbers – After fertilization single cell zygote gives rise to all cells in adult
Germ-Line Tissues
-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
What are the 3 unique features of Meiosis?
- Synapsis
- Homologous Recombination
- Reduction Division
Synapsis
– 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.
Homologous Recombination
– Genetic exchange occurs between
the homologous chromosomes
while joined
– Called CROSSING OVER
Reduction Division
– Chromosomes do not replicate
between the two nuclear divisions.
5 Stages of Prophase I?
- Leptotene
- Chromosomes condense tightly - Zygotene
-A lattice of protein laid down between
homologues in synapsis –called synaptonemal complex - Pachytene
-After synapsis , complex keeps
chromosomes and genes exactly across from each
other – DNA duplex unwinds and makes links with
complementary strands of homologue - Diplotene
-Protein lattice complex breaks down –
intense cell growth occurs – chromosomes decondense
and become active in transcription - Diakinesis
-Transition into metaphase, transcription
ceases and chromosomes recondense
Synaptonemal complex
A lattice of protein laid down between
homologues in synapsis
CROSSING OVER
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
Metaphase I
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
Anaphase I
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.
After Meiosis I:
sister chromatids are not identical.
The Second Meiotic division.
• 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.
Asexual reproduction
– 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
Budding off and Fragmentation
Multicellular organisms
– May reproduce by budding off localized masses of cells
– Sponges reproduce asexually by fragmentation.
Parthenogenesis
Development from an unfertilized egg.
Example: bees
– Fertilized eggs become diploid females
– Unfertilized eggs become haploid males
The evolutionary consequences of sex.
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
Evolutionary Process Is Revolutionary and Conservative
– 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
Conservative process in asexual organisms
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
What increases pace of evolution?
Pace of evolution increased with greater genetic
diversity.
Genetic diversity is raw material of evolution
What is the evolutionary rationale for sexual
reproduction?
• 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
