Cell Division - Meiosis

Why is Meiosis so Important? Meiosis is responsible for all of us inheriting genes from our parents It is the process by which gametes (sex cells – eggs & sperm) are formed It is the process by which we get a mix of characteristics from mom and from dad

Principles of Genetics Each organism must inherit a single copy of every gene from its “parents” When an organism produces its own gametes, those two sets of genes must be separated from each other so that each gamete contains only one set of genes Remember, genes are located on chromosomes inside the cell’s nucleus

Chromosome Number Let’s consider the fruit fly… A somatic cell in an adult fruit fly has 8 chromosomes 4 of the chromosomes came from the fruit fly’s male parent, and 4 came from the female parent These two sets of chromosomes are homologous – meaning that each of the 4 chromosomes that came from the male parent has a corresponding chromosome (and therefore corresponding genes) from the female parent

Chromosome Number A cell that contains both sets of homologous chromosomes is said to be diploid, which means “two sets” The number of chromosomes in a diploid cell can be represented by the symbol 2N **(think 2, like 2 sets) So for the fruit fly, the diploid number is 8, which can be written as 2N = 8 But the gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes – these are called haploid, which means “one set” The haploid number can symbolized as just N **(think 1 N, like 1 set) So for the fruit fly, the haploid number is 4, which can be written as N = 4

Meiosis How are haploid (N) gamete cells produced from diploid (2N) cells? That’s where meiosis comes in! Meiosis is a process in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell Meiosis involves two distinct divisions, called Meiosis I and Meiosis II By the end of Meiosis II, the diploid cell that entered meiosis has become four haploid cells

Gamete Formation In males, the four haploid cells produced at the end of Meiosis are called sperm In females, only one of the four cells produced is involved in reproduction, and it is called an egg

Phases of Meiosis – Meiosis I Description Interphase I Cells undergo a round of DNA replication, forming duplicate chromosomes Prophase I Each chromosome pairs with its corresponding homologous chromosome and crossing over occurs (Crossing Over: homologous chromosomes exchange portions of themselves to produce new combinations of genes) Metaphase I Chromosomes line up down the middle and spindle fibers attach to them

Crossing Over

Phases of Meiosis – Meiosis I Description Anaphase I Homologous pairs separated and are pulled to the poles of the cell Telophase I Nuclear membrane reforms, cytoplasm is still connected Cytokinesis I A cleavage furrow separates it into two haploid (N) daughter cells

Phases of Meiosis – Meiosis II Description Prophase II Similar to other prophases – nuclear membrane disintegrates and spindle fibers begin to form Metaphase II The haploid number of chromosomes line up down the middle and the spindle fibers attach Anaphase II Sister chromatids separate and move towards the poles of the cell

Phases of Meiosis – Meiosis II Description Telophase II Similar to other telophases – nuclear membrane begins to reform and cytoplasm is still connected Cytokinesis II Meiosis II results in 4 haploid daughter cells

Mitosis vs. Meiosis Mitosis Meiosis Homologous chromosomes do not pair up Crossing Over occurs One cell division Four daughter cells Daughter cells are diploid (2N)

Mitosis vs. Meiosis Mitosis Meiosis Homologous chromosomes do not pair up Homologous chromosomes pair up Crossing Over occurs One cell division Four daughter cells Daughter cells are diploid (2N)

Mitosis vs. Meiosis Mitosis Meiosis Homologous chromosomes do not pair up Homologous chromosomes pair up Crossing over does not occur Crossing Over occurs One cell division Four daughter cells Daughter cells are diploid (2N)

Mitosis vs. Meiosis Mitosis Meiosis Homologous chromosomes do not pair up Homologous chromosomes pair up Crossing over does not occur Crossing Over occurs One cell division Two cell divisions Four daughter cells Daughter cells are diploid (2N)

Mitosis vs. Meiosis Mitosis Meiosis Homologous chromosomes do not pair up Homologous chromosomes pair up Crossing over does not occur Crossing Over occurs One cell division Two cell divisions Two daughter cells Four daughter cells Daughter cells are diploid (2N)

Mitosis vs. Meiosis Mitosis Meiosis Homologous chromosomes do not pair up Homologous chromosomes pair up Crossing over does not occur Crossing Over occurs One cell division Two cell divisions Two daughter cells Four daughter cells Daughter cells are diploid (2N) Daughter cells are haploid (N)