Question

What is each step in meiosis? Are there any examples of what can happen if an error occurs?

Answer 1

MEIOSIS-It is special type of cell division which is present in sexual reproducing oragnism for producing gametes examples sperm and egg.In meiosis one cell divides twice to form four daughter cells

Meiosis can be divided into two main stages:

(a) Meiosis-I

(b) Meiosis-I

Meiosis-I and Meiosis-II occure one after the another.

                              Meiosis-I

It further divided in to four phases:

(a)Propase-I Most complicated and prolonged phase

Prophase-I subdivided into five phases,Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

(i)Leptotene- chromosomes become gradually visible

(ii) Zygotene-Chromosomes start pairing together, Formation of synapsis, formation of synaptonemal complex

(iii)Pachytene- chromosomes clearly appear as tetrads, crossing over takes place between non-sister chromatids of the homologous chromosomes.

(iv)Diplotene-Synapotnemal complex is dissolved tetrad remin clearly visible, X-shaped structures formed are called chiasmata

(v)Diakinesis-final stage of prophase-I,chromosomes become fully condensed.

(b)Metaphase-I- The bivalent chromosomes are arrange on the equatorial plate.

(c)Anaphase-I-Homologous chromosomes break their connection with each other.

(d)Telophase-I-Homologous chromosomes reach their poles.reappear nuclear membrane and nucleolus.

                                              MEIOSIS II

(i)Prophase II: very short stage,. Nuclear membrane disappears. Chromosomes again become compact.

(ii)Metaphase-II: chromosomes align at the equator. Spindle fibres from the opposite poles get attached to the kinetochores of sister chromatids.

(iii)Anaphase -II:Centromeres split ,sister chromatids move towards the opposite poles.

(iv)Telophase II:   Chromosomes complete their move to the opposite poles of the cell,chromosomes get enclosed by nuclear envelope,resulting in the formation of four daughter cells.

If error occurs in meiosis some exmaples are :

trisomy 21 extra copy of chromosome disorder name in down syndrome,Klinefelter syndrome,