Meiosis Flashcards
Meiosis intro
• Parents endow offspring with encoded information called genes
• Genes program the specific traits that emerge as we develop from fertilized eggs into adults
• In animals and plants, reproductive cells called gametes are the vehicles that transmit genes from one generation to
the next
• A gene’s specific location along the length of a chromosome is called the gene’s locus (loci)
Asexual Reproduction
• Only organisms that reproduce asexually have
offspring that are exact genetic copies of
themselves
• Asexual reproduction involves:
o One parent
o Transfer of all its genes to offspring
o No fusion of gametes
o Gives rise to clones
• Genetic differences only arise from mutations
Sexual Reproduction
• Two parents give rise to offspring that have unique
combinations of genes inherited from two parents
• In contrast to clones, offspring of sexual
reproduction vary genetically from siblings and
parents
• Involves:
o Two parents
o Transfer of genes from both parents
o Fusion of gametes
Human karyotype
• A set of chromosomes in a cell is called a karyotype
• It shows the number, size and shape of the chromosomes during metaphase of mitosis
• In human karyotype, non-sex chromosomes (autosomes) are grouped together in pairs and placed in groups A-G
• Each chromosome of a pair have the same length, centrosome position and staining pattern à homologous
chromosomes or homologs
• Sex chromosomes, gonosomes, (X-female, Y-male) are placed separately
• Karyotypes are useful as they show:
o Whether a cell comes from a male or female
o Abnormal chromosomes
Meiosis
• Meiosis is cell division that takes place in the reproductive organs of
both plants and animals and produces:
o Gametes – animals
o Spores – plants
• In meiosis the number of chromosomes is reduced from two sets (2n)
in the parent cell to one set (n) each of the daughter cells formed
• The gametes/spores formed are called haploid as they only have one
set of chromosomes
• In sexual reproduction a male haploid gamete fuses with female
haploid gamete during fertilization to form a diploid zygote
• Animals – testes & ovaries
o Formation of sperm – spermatogenesis
o Formation of eggs – oogenesis
• Plants – sporangia in seed-bearing plants
o Microsporangia – pollen sacs in anthers
o Mega sporangia – ovules on female ovaries
How does meiosis take place?
• DNA of parent cell is replicated in interphase
• This precedes both meiosis and mitosis
• However, meiosis replication is followed by two divisions
o Meiosis 1 – reduction division which results in two cells forming with half number of chromosomes – haploid
o Meiosis 2 – copying division where two haploid cells are divided again by mitosis to form 4 haploid cells
Prophase I
• Chromatin network condenses
• Separate chromatids (although replication has occurred)
cannot be distinguished
• Homologous chromosomes arrange themselves in pairs
• Two chromosomes of each homologous pair lie along
side each other to form a bivalent
• Each chromosome of the bivalent splits longitudinally into
two identical chromatids called sister chromatids
• Crossing over of the chromatids takes place
• Nuclear membrane and nucleolus disappear
• Centrosomes duplicates and centrioles move to opposite
poles
• Spindle fibres develop between the centrioles
Metaphase I
• Bivalents position themselves on equator
• Chromosomes lie in a double row on either side of
equator
• Some spindle fibres attach to centromeres
Anaphase I
• Spindle fibres contract and shorten pulling chromosomes
towards poles
• Homologous chromosomes are separated
• One chromosome of bivalent moves to one pole and the
other chromosome to the other pole
• Cytokinesis begins
NB: no division of centromeres occurs. Therefore, chromosomes
move to poles and not chromatids. Separation of homologous
pairs results in chromosome number halving
Telophase I
• Chromosomes group at poles
• Spindle fibers disappear
• Nuclear membrane and nucleolus form
• Invagination occurs to form two daughter cells
• Each daughter cell has one chromosome from each
homologous pair – haploid number
• Chromosome is double stranded with recombinant
chromatids
Crossing Over
The mutual exchange of pieces of chromosomes. Groups of genes are swapped between maternal and paternal chromosomes • When homologous pairs come together (synapsis) to form bivalents, they swap pieces of their inner chromatids – thus breaking and reforming their DNA • Points of crossing over where chromatids break are called chiasmata • In this way some genes of paternal chromatid change place with that of maternal chromatid • Forms a recombinant chromatid • Outer unchanged chromatids are called parentals Importance • Exchange produces unique combination of genes and increases variation • During exchange process mistakes may occur leading to mutation – sometimes beneficial in influencing evolution
Prophase II
• Each chromosome consists of two chromatids and
centromere
• The nucleolus a nuclear membrane disappears
• Spindle forms consisting of centrioles at opposite poles
Metaphase II
• The chromosomes move to the equator and align in a single
row
• Some spindle fibres attach to the centromeres
Anaphase II
- The centromere of each chromosome splits into two
- The two chromatids separate and move to opposite poles
- Chromatids at poles are known as daughter chromosomes
- Cytokinesis begins
Telophase II
• Daughter chromosomes group at poles • Chromosomes in each daughter cell are single stranded with recombinant chromatids • Spindle fibres disappear • Nuclear membrane and nucleolus forms • Invagination occurs • 4 daughter cells have formed each with haploid number of chromosomes • Due to crossing over genetic information in each cell is not identical • Daughter cells in male = sperm • Female = eggs
