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CGHS Biology - Meiosis: Making of Sex Cells
Transcript of CGHS Biology - Meiosis: Making of Sex Cells
Why sex happens?
Meiosis - Cuts the number in chromosome in half
Meiosis looks similar to mitosis, it is really very different
Steps in Meiosis
Meiosis begins like mitosis does, with replication of DNA in S phase of interphase
Involves two cell divisions instead of just one Meiosis I and Meiosis II
Role of Meiosis in Sex Ratio
Humans have 22 pairs of autosomes and one pair of sex chromosomes; females have two X chromosomes, but males have one X and one Y.
Is a haploid mitotic division
Is when sister chromatids separate
Advantages of Sex
Introduces new combinations of heritable traits in the offspring
When homologues separate and
Chromosomes number cut in half
Meiosis Lecture Spring Semester
Not found in prokaryotes
Asexual reproduction only
Often an option for single-celled eukaryotes (e.g. yeast)
Usually a requirement for multi-celled eukaryotes
Most are absolutely dependent on sexual reproduction
Must keep up with the rapid evolution of parasites, bacteria, and viruses
These organisms can evolve very quickly because they have enormous numbers of offspring
Asexual reproduction has the advantage of being very rapid
Diploid vs. Haploid
n = number of different chromosomes
Human body cells are diploid = 2 of each chromosomes (homologues) 2N.
Must has a way to reduce the chromosomes in half
Cells with half the number of chromosomes is called haploid
These cells are called sex cells
(sperm and eggs)
Genetic variation created by “scrambling up”
chromosomes through two processes:
Crossing over of homologous pairs during prophase of MI
Sex chromosomes pair like homologues and separate in meiosis I, so each haploid cell that results has either an X or a Y in males.
The sex of a child is therefore determined by the sperm at fertilization
Spermatogonia: diploid cells that can divide by mitosis to make more of themselves, or make cells destined to become sperm
Makes 4 Sperm Cells
Process to make oocytes (ova or egg cells).
Makes one egg cell and 3 polar bodies (undeveloped egg cells that will degrade).
How is meiosis different than mitosis?
In mitosis, chromosomes line up individually, while in meiosis, the two chromosomes in each homologous pair line up next to each other. This pairing process is called synapsis.
a tetrad in reference to the four sister chromatids involved.
Mitosis = chromosomes lined up in the middle
This is a single file line
Meiosis = sister chromologues lined up
Side by side or in pairs
Independent assortment of homologue pairs during metaphase of MI
Means chromosomes do not have to stay together,
but are independent of each other
You can tell it is Meiosis II, because there are two cells
failure of homologous chromosomes or sister chromatids to separate subsequent to metaphase in meiosis so that one daughter cell has both and the other neither of the chromosomes
Examples of non-disjunction diseases: