form 4 biology chap5 pt2
DESCRIPTION
TRANSCRIPT
LEARNING OUTCOMES• To state the necessity of trait inheritance in
offspring for continuation of life,• To state the necessity to maintain diploid
chromosomal number from generation to generation,
• To state the necessity for production of haploid gametes in sexual reproduction,
• To state the significance of meiosis,• To identify the type of cell that undergoes
meiosis,• To explain the process of meiosis,• To arrange the various stages of meiosis in the
correct order,• To compare and contrast meiosis I & meiosis II,• To compare and contrast meiosis & mitosis.
MEIOSIS• A division of the nucleus to produce 4 daughter
cells each containing half the chromosome number of the parent nucleus.
• A type of cell division - occurs in reproduction organs to produce 4 daughter cells called gametes.
• The process of nuclear division that reduces the number of chromosomes in new cells to half the number of chromosomes in the parent cell.
• Each daughter cell receives 1 set of chromosome from each pair of homologous chromosomes – haploid cell (n)
MEIOSIS• Genetic material in the gametes differs
from the parent cell & each other
• Animals : testes in males – sperm ovaries in females – ovum
• Plants : anthers – pollen (male gametes)ovaries – ovules (egg cells)
• Consists of Meiosis I and Meiosis II
THE SIGNIFICANCE OF MEIOSIS
• To allow trait inheritance in offspring – the transmission of traits
• To maintain diploid number in each generation - the organisms are always diploid
• To ensure the production of haploid gametes in sexual reproduction
• To produce genetic variation among offsprings
MEIOSIS• Meiosis I : the homologous chromosomes
are separated into two haploid daughter cells
• Meiosis II : the sister chromatids of each daughter cell are separated.
• Meiosis II = mitosis in a haploid cell
• All chromosomes are replicated during interphase sister chromatids joined at centromere. The chromosomes do not replicate again between Meiosis I & Meiosis II
MEIOSIS IPROPHASE I
• Centrioles move to opposite poles.• Chromosomes thicken & condense• The homologous chromosomes form
bivalent through synapsis• Crossing over happened : non-sister
chromatids exchange segments of DNA new combination of genes on a chromosome
• Chiasmata – the points at which segments of chromatids cross over
• Nucleus membrane & nucleolus disappear• Spindle fibres are formed
METAPHASE I• The paired chromosomes are lined
up at the equator of the cell
• One chromosome of each pair faces each pole of the cell
• the chromosomes attached to the spindle fibres at their centromere
• The centromere does not divide
ANAPHASE I• The paired chromosomes separate from
one another & move to opposite poles
• The spindle fibres pulled one chromosome of each pair to each pole
TELOPHASE I• The chromosomes arrive at the poles.
• Each pole has a haploid daughter nucleus (contain one set)
• The spindle fibres disappear, the nuclear membrane & nucleolus reappears in each nucleus.
• Cytokinesis occurs
• No interphase, no replication of chromosomes
MEIOSIS IIPROPHASE II
• Centriole duplicates & move to opposite pole
• Nuclear membrane disappear, spindle fibres re-form
• The chromosomes move to the equator of the spindle.
METAPHASE II• Chromosomes lined
up at the equator of the cell facing opposite poles
• Each sister chromatid is attached to the spindle fibres at the centromere
ANAPHASE II• The sister chromatids
together split
• Chromatids separate
• Spindle fibres pull each chromatid to opposite poles
• Centromere are divided
TELOPHASE II• Chromatids reach the poles &
become new chromosomes
• Nuclear membrane & nucleolus form again at each chromosome
• Chromosomes become extended & not visible
• Cytokinesis occurs & 4 haploid daughter cell are formed.
THE COMPARISON BETWEEN MEIOSIS I & MEIOSIS II
MEIOSIS I SIMILARITIES MEIOSIS II
•Consist of 4 stages : P,M,A,T•Involve division of nucleus & cytokinesis
DIFFERENCES
Occur Synapsis Not occur
Yes Cross over No
MI-paired homologous chromosomes line up
at the equator
Metaphase MII – each chromosome with sister chromatids line up at the equator
AI – paired homologous chromosomes separate
& move to opposite poles
Anaphase AII – the sister chromatids separate & move to opposite poles
2 haploid cells are formed
At the end 4 haploid cells are formed
THE COMPARISON BETWEEN MITOSIS & MEIOSIS
MEIOSIS SIMILARITIES MITOSIS•Division of cells
•The chromosomes replicates only once
DIFFERENCES
In reproductive organ Place occur In somatic cell
Parent cell divides twice Number of divisions Parent cell divides once
Four haploid daughter cells Number of daughter cells Two diploid daughter cells
Occurs during prophase I Synapsis of homologous chromosomes
Not occurs
Occurs twice Number of cytokinesis Occurs once
Occurs during prophase I Crossing over of chromatids of homologous chromosomes
Not occurs
Half of number of chromosomes of the parent cell (haploid)
Number of chromosomes in daughter cells
Genetically identical to the parent cell
Genetically non-identical to the parent cell & each other
Genetic composition of daughter cells
Genetically identical to the parent cell
Occurs once during interphase before meiosis I
DNA replication Occurs during interphase before mitosis begins