meiosis. what is meiosis and why is it important? to really understand basic concepts of genetics,...
TRANSCRIPT
Meiosis
What is Meiosis and Why is it Important?
• To really understand basic concepts of genetics, you need to know how hereditary traits are passed from parents to offspring.– Sexual reproduction – 2 parts:
• Fertilization – genetic material from parents brought together to form new genetic identity of offspring
• Meiosis – nuclear division helping to diversify genetics of the offspring
• Occurs in sex cells – sperm and egg cells
The Basics
• Genes located on chromosomes in the cell nucleus• Chromosomes – each species has certain chromosome
numbers per somatic (body) cells which are diploid (2n)– Cats – 38, humans – 46, goldfish – 94, corn plants – 10– In every diploid cell, each chromosome has a partner resembling
each other = homologous pairs/homologues. One homologue comes from one parent and the other comes from the other parent
– Diploid cells contain two complete sets of chromosomes and two complete sets of genes – one from “mom” and one from “dad”
The Basics (continued)
• Sex cells (gametes – egg and sperm) are “special” in that they have exactly half the number of chromosomes as the somatic cells – thus, sex cells are haploid (n)– Gametes have one allele for each gene– Cats – 19, humans – 23, goldfish – 47, corn plants – 5– Why haploid?
• When a male gamete (sperm) fuses with a female gamete (egg), the diploid number is restored = counterbalances effects of fertilization
The Phases of Meiosis
• There are two nuclear divisions during meiosis:– Meiosis I– Meiosis II
• Meiosis forms gametes and is a process where the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell – 2n to n
Overview of Meiosis
Stages in Meiosis – Meiosis I
• Meiosis I – homologous chromosomes pair and separate– Interphase – chromosomes replicate– Prophase I – chromosomes condense and each chromosomes
pairs with its homologue, forming a tetrad (tetrads – 4 chromosomes). Once contact is made at any point, crossing over (exchanging genetic information) can occur = VERY important for genetic diversity
– Metaphase I – paired homologues line up in middle of cell– Anaphase I – paired homologues separate to opposite ends of
the cell (tetrad separates)– Telophase I – two genetically different daughter cells form
Section 11-4
Crossing-Over
Go to Section:
A Closer Look at Crossing Over
Section 11-4
Crossing-Over
Go to Section:
A Closer Look at Crossing Over
Section 11-4
Crossing-Over
Go to Section:
A Closer Look at Crossing Over
Stages in Meiosis – Meiosis II
• Meiosis II – chromatids of each homologue separate – no replication occurs, which allows for reduction in number of chromosomes– Prophase II – cell prepares for Metaphase II (spindles
form, nuclear envelope disappears)– Metaphase II – chromatid pairs line up in middle of
cell– Anaphase II – chromatids separate into individual
chromosomes– Telophase II – 4 haploid cells form, each genetically
different from one another and from the parent cell
Meiosis I
Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.
Section 11-4
Figure 11-15 Meiosis
Go to Section:
Meiosis I
Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.
Section 11-4
Figure 11-15 Meiosis
Go to Section:
Meiosis I
Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.
Section 11-4
Figure 11-15 Meiosis
Go to Section:
Meiosis I
Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.
Section 11-4
Figure 11-15 Meiosis
Go to Section:
Meiosis II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Section 11-4
Figure 11-17 Meiosis II
Go to Section:
Meiosis II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Section 11-4
Figure 11-17 Meiosis II
Go to Section:
Meiosis II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Section 11-4
Figure 11-17 Meiosis II
Go to Section:
Meiosis II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Section 11-4
Figure 11-17 Meiosis II
Go to Section:
Meiosis II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Section 11-4
Figure 11-17 Meiosis II
Go to Section:
Meiosis
Meiosis
Gamete Formation
• In males – haploid gametes = sperm
• In females – haploid gametes = egg
Because of Meiosis…
• We can get a great deal of genetic diversity– Random Fertilization – 1 egg and 1 sperm
each with such different genetic combinations– Crossing Over – Prophase I, different
combinations of parts of chromosomes– Independent Assortment – what “side of cell”
each chromosome is on – Mendel’s 2nd Law
Independent Assortment
Meiosis vs. Mitosis
• Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells
• Mitosis allows for multicellular organisms to grow
• Meiosis produces gametes with many different genetic possibilities
