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Figure 34. 
Sexual Reproduction
 
3.1 Reading Resource # 2:
Meiosis I 
Meiosis, also known as nuclear division in haploid cells, is connected to mitosis. You now know that 
the cell reproduction cycle known as mitosis produces daughter nuclei that are identical to their parent nuclei 
genetically. In mitosis, the number of chromosomes set in the parent and daughter nuclei is the same diploid 
for most plants and animals. Meiosis employs many of the same mechanisms as mitosis. The nuclei that emerge 
at the conclusion of a meiotic cell division, however, are always haploid and the starting nucleus is always 
diploid. Meiosis consists of one round of chromosome duplication and two rounds of nuclear division to reduce 
the number of chromosomes. 
The same stage names are given to each of the division stages because the events that take place during 
those stages are comparable to those that occur during mitosis. However, because there are two rounds of 
division, the stages are designated with a “I” or “II.” Thus, meiosis I is the first round of meiotic division and 
consists of prophase I, prometaphase I, and so on. 
Meiosis I
reduce the number of chromosomes sets from two 
to one. Genetic information is also mixed during this division to create unique recombinant chromosomes. 
Meiosis II
, in which the second round of meiotic division takes place in a way that is like mitosis, includes 
prophase II, prometaphase II, and so on. 
Interphase 
The G1, S, and G2 phases of the interphase, which precedes meiosis, are very similar to the phases that 
come before mitosis. The first interphase phase, G1, is primarily concerned with cell growth. The DNA of the 

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chromosomes is replicated during the S phase. The cell makes its final preparations for meiosis during the G2 
phase.
Each chromosome develops two identical copies (known as sister chromatids) during DNA duplication 
of the S phase, which are held together at the centromere until they are separated during meiosis II. The 
centrosomes that arrange the microtubules of the meiotic spindle replicate in an animal cell as well. The cell is 
now ready to enter the initial meiotic phase. 
Meiosis I 
The chromosomes can be observed under a microscope early in prophase I. The proteins linked to 
homologous chromosomes bring the pair together as the nuclear envelope starts to degrade. The tight pairing 
of the homologous chromosomes is called 
synapsis
. In synapsis, the genes on the chromatids of the 
homologous chromosomes are precisely aligned with each other. An exchange of chromosome segments 
between non-sister homologous chromatids occurs and is called