Mendel Lecture Tsering Kalden
Meiosis I (reduction) and Meiosis II (division)
Meiosis produces 4 haploid cells
In a process called synapsis, special proteins attach homologous chromosomes
Brief summary of the Process:
homologous chromosome , replication occurs during premiotic interphase, each
chromosome consists of 2 sister chromatids, chromosomes pair during prophase I
for meiosis, may undergo recombination , first meiotic division separates the
holmologs, placing one in each of the two cells resulting form the division. the
products have haploid number of chromosome, each chromosomes still contain 2
chromatids.the second meiotic division occurs, separates the sister chromatids,
one in each cell ( 4 cells are produced) as a final product of meiosis.
key terms and ideas:
Haploid = 1n = cells with one complete set of chromosomes, i.e unfertilized egg
somatic cells: have 23 choromosmes
synapsis: process where special proteins attach homologous chromosomes tightly
Haploid sperm fuses with haploid egg to get diploid zygote (fertilized egg)
• Male parent provides 23 chromosomes (1 set) and female parent 23
chromosomes (1 set)
Diploid = 2n = cells with two complete sets of chromosomes i.e. almost all body
cells of human are diploid, diploid human cells have 46 chromosomes, have a pair
of homologous chromosome pairs.
SexualReproduction: new individual gets 1⁄2 genetic information from one parent
& 1⁄2 from other parent, offspring are genetically different ( greater variation)
Asexual Reproduction: clones, genetically alike offspring takes copies of one
parent, no variation.
Homologous Chromosomes: are same size, same gene loci (gene location),
different alleles (alternate forms of a gene), a diploid nucleus contains two sets of
homologs (2n) one from each parent.
Human females have a true homologous pair XX
Each gene occupies a specific “locus” on a specific chromosome
Human males XY – X and Y are different, Y (X and Y are called sex chromosomes)
Autosomes = all other non-sex chromosomes
interphase:chromosomes are replicated to form sister chromatids (genetically
identical) joined at the centromere, single centrosome is also replicated
prophase I:chromosomes condense, homologous chromosomes pair up to form
– synapsis occurs and then recombinations at several sites of the chromatid
crossed (chiasmata) and segments of the chromosomes are traded. A spindle
forms from, spindle fibers attached to kinetochores on the chromosomes begin to
move the tetrads around.
metaphase 1: the tetrads are all arranged at the metaphase plate, microtubules
attached to the kinetochore
anaphase 1: the homologous chromosomes separate and are pulled to opposite
In telophase I, movement of homologous chromosomes continues until there is a
haploid set at each pole.