Meiosis and Sexual Reproduction Meiosis and Sexual Reproduction Production of Haploid Cells Variation in Genetic Material

Sperm and Egg Sperm trying to enter egg and leave packet of genetic information – i.e. chromosomes containing DNA. Sperm trying to enter egg and leave packet of genetic information – i.e. chromosomes containing DNA. Only one sperm will be successful Only one sperm will be successful For zygote to have same number chromosomes as parents, chromosome number must be halved when egg and sperm made For zygote to have same number chromosomes as parents, chromosome number must be halved when egg and sperm made Halving chromosome number done by MEIOSIS (produces haploid cells) Halving chromosome number done by MEIOSIS (produces haploid cells) Diploid number restored when sperm fertilizes egg Diploid number restored when sperm fertilizes egg

Review What are the following? Evolution Natural selection Homologous chromosomes Chromatids Answers Change in genetic makeup of species over time. Change in genetic makeup of species over time. Individuals with favorable traits are more likely to survive and reproduce Individuals with favorable traits are more likely to survive and reproduce Chromosomes similar in size, shape and genetic makeup Chromosomes similar in size, shape and genetic makeup Copied chromosomes still attached by centromere Copied chromosomes still attached by centromere

Review What are the following? Haploid Cells Haploid Cells Diploid Cells Diploid Cells Spindle Fibers Spindle Fibers The steps of mitosis The steps of mitosis Answers Cells with one set of chromosomes Cells with two sets of chromosomes A structure made of microtubules - helps organize and separate chromosomes Prophase, Metaphase, Anaphase, Telophase

What would happen if the number of chromosomes weren’t reduced OrganismDiploid Chrom. Number # after 1 generation # after 2 generations # after 3 generations # after 4 generations Fruit Fly Garden Pea Horse

Formation of Haploid Cells MMMM EEEE IIII OOOO SSSS IIII SSSS

Meiosis I 1 st Meiotic Division 1 st Meiotic Division

Before Meiosis 1, DNA replicated 1 st step of Meiosis 1 Chromosomes condense Chromosomes condense Watch it coil! Watch it coil! Watch it coil! Watch it coil! Nuclear membrane breaks down Nuclear membrane breaks down Homologous chromosomes pair up Homologous chromosomes pair up Crossing over (parts of chromatid on one homologous chromosome exchanged with corresponding part on other homologous chromosome) Crossing over (parts of chromatid on one homologous chromosome exchanged with corresponding part on other homologous chromosome) Animation of Prophase 1 Animation of Prophase 1 Animation of Prophase 1 Animation of Prophase 1 Prophase 1

Meiosis 1, continued 2 nd phase of Meiosis 1 Pairs of homologous chromosomes are moved by spindle fibers to cell equator Pairs of homologous chromosomes are moved by spindle fibers to cell equator Homologous chromosomes remain together Homologous chromosomes remain together This is when independent assortment occurs This is when independent assortment occurs Animation of Metaphase 1 Animation of Metaphase 1 Animation of Metaphase 1 Animation of Metaphase 1 Metaphase 1

Meiosis 1, continued 3 rd phase of Meiosis 1 Homologous chromosomes separate Homologous chromosomes separate Chromosomes of each pair are pulled to opposite poles by spindle fibers Chromosomes of each pair are pulled to opposite poles by spindle fibers CHROMATIDS DO NOT SEPARATE AT THEIR CENTROMERES! CHROMATIDS DO NOT SEPARATE AT THEIR CENTROMERES! Each chromosome made of two RECOMBINED chromatids Each chromosome made of two RECOMBINED chromatids Animation of Anaphase 1 Animation of Anaphase 1 Animation of Anaphase 1 Animation of Anaphase 1 Anaphase 1

Meiosis 1, continued 4 th stage of Meiosis 1 Individual chromosomes gather at each of the poles Individual chromosomes gather at each of the poles Cytoplasm divides Cytoplasm divides Both cells contain 1 chromosome from each pair of homologous chromosomes Both cells contain 1 chromosome from each pair of homologous chromosomes Animation of Telophase 1 and cytokinesis Animation of Telophase 1 and cytokinesis Animation of Telophase 1 and cytokinesis Animation of Telophase 1 and cytokinesis Telophase 1 and cytokinesis

Meiosis II 2 nd Meiotic Division 2 nd Meiotic Division

Meiosis II 1 st Phase of Meiosis II New spindle fibers form around the chromosomes New spindle fibers form around the chromosomes Animation of Prophase II Animation of Prophase II Animation of Prophase II Animation of Prophase II Prophase II

Meiosis II, continued 2 nd phase of Meiosis II Chromosomes line up along the equator and are attached at their centromeres to spindle fibers Chromosomes line up along the equator and are attached at their centromeres to spindle fibers Animation of Metaphase II Animation of Metaphase II Animation of Metaphase II Animation of Metaphase II Metaphase II

Meiosis II, continued 3 rd phase of Meiosis II Centromeres divide Centromeres divide Chromatids (now called chromosomes) move to opposite poles of cell Chromatids (now called chromosomes) move to opposite poles of cell Animation of Anaphase II Animation of Anaphase II Animation of Anaphase II Animation of Anaphase II Anaphase II

Meiosis II, continued 4 th phase of Meiosis II Nuclear membrane forms around each set of chromosomes. Nuclear membrane forms around each set of chromosomes. Spindle fibers break down Spindle fibers break down Cells undergo cytokinesis Cells undergo cytokinesis There are now 4 haploid cells There are now 4 haploid cells Animation of Telophase II and cytokinesis Animation of Telophase II and cytokinesis Animation of Telophase II and cytokinesis Animation of Telophase II and cytokinesis Telophase II and cytokinesis

IMPORTANT! The purpose of meiosis is to produce cells with half the genetic material as the parent cell The purpose of meiosis is to produce cells with half the genetic material as the parent cell Meiosis II Meiosis I Parent Cell Diploidhaploid

IMPORTANT! The purpose of mitosis is to produce cells with same amount of genetic material as the parent cell The purpose of mitosis is to produce cells with same amount of genetic material as the parent cell Mitosis Parent Cell Diploid

Meiosis and Genetic Variation Independent Assortment & Crossing Over  Genetic Variation

Meiosis = rapid generation of new genetic combinations How genetic variation arises Independent Assortment Independent Assortment Independent Assortment Independent Assortment Crossing Over Crossing Over Crossing Over Crossing Over Random Fertilization – Random combination of 2 gametes Random Fertilization – Random combination of 2 gametes Why don’t they all look alike?

Why is Genetic Variation Important? Evolution! Evolution! Pace of evolution increases as genetic variation increases Pace of evolution increases as genetic variation increases Genetic recombination increases evolution Genetic recombination increases evolution 3 rd type is produced – not identical to either parent 3 rd type is produced – not identical to either parent Genetic variation can bring out good traits or not so successful traits Genetic variation can bring out good traits or not so successful traits

Gamete Formation Sperm production = spermatogenesis – meiosis results in production of 4 sperm Sperm production = spermatogenesis – meiosis results in production of 4 sperm Egg production = oogenesis – meiosis results in production of 1 egg cell and 3 polar bodies because cytoplasm divides unequally Egg production = oogenesis – meiosis results in production of 1 egg cell and 3 polar bodies because cytoplasm divides unequally Polar bodies die Polar bodies die Egg lives Egg lives Egg = Ovum Egg = Ovum

Modeling Crossing Over Each person in group needs 4 paper strips – 2 of one color, 2 of another color – taped together (loosely) to represent chromatids joined by centromeres 4 paper strips – 2 of one color, 2 of another color – taped together (loosely) to represent chromatids joined by centromeres Pen or pencil Pen or pencil Scissors Scissors Tape Tape A textbook (you should end up with 4 textbooks in a group) A textbook (you should end up with 4 textbooks in a group) Procedure On two strips of the same color, write “A”, “B”, and “C” – one of the two homologous chromosomes On two strips of the same color, write “A”, “B”, and “C” – one of the two homologous chromosomes On two strips of another color, write “a”,“b”, “c” this is another the two homologous chromosomes On two strips of another color, write “a”,“b”, “c” this is another the two homologous chromosomes Use scissors and tape to demonstrate crossing over between chromatids of homologous chromosomes Use scissors and tape to demonstrate crossing over between chromatids of homologous chromosomes

Modeling Crossing Over What do the letters “A”, “B” and “C” and “a”, “b”, and “c” represent? What do the letters “A”, “B” and “C” and “a”, “b”, and “c” represent? Why are the chromosomes you made homologous? Why are the chromosomes you made homologous? Compare the number of types of different chromatids (combinations of “A”, “B”, “C”, “a”, “b”, and “c”) before crossing over and after crossing over. Compare the number of types of different chromatids (combinations of “A”, “B”, “C”, “a”, “b”, and “c”) before crossing over and after crossing over. How does crossing over relate to genetic recombination? How does crossing over relate to genetic recombination? During what phase of meiosis does crossing over occur? During what phase of meiosis does crossing over occur?

Modeling Independent Assortment In your groups of 4, line up homologous pairs as the chromosomes would line up during Metaphase I. Do this on top of two textbooks In your groups of 4, line up homologous pairs as the chromosomes would line up during Metaphase I. Do this on top of two textbooks How many different arrangements can you make? How many different arrangements can you make? What will happen during Anaphase 1? Will the 2 cells which are the result of Meiosis one contain the same genetic information? What will happen during Anaphase 1? Will the 2 cells which are the result of Meiosis one contain the same genetic information? Pull books apart to represent cytokinesis Pull books apart to represent cytokinesis How does independent assortment relate to genetic recombination How does independent assortment relate to genetic recombination

Modeling Meiosis II You should now have 2 cells – each with 4 double chromosomes in them (there will be more or less chromosomes, depending on how many people are in your group) You should now have 2 cells – each with 4 double chromosomes in them (there will be more or less chromosomes, depending on how many people are in your group) Line up double chromosomes as in Metaphase II. Use another textbook to add to your “cytoplasm”. Line up double chromosomes as in Metaphase II. Use another textbook to add to your “cytoplasm”. Snip tape between the centromeres so that the single chromosomes can travel to opposite poles. Pull books apart to model cytokinesis Snip tape between the centromeres so that the single chromosomes can travel to opposite poles. Pull books apart to model cytokinesis Do the same thing with your second cell Do the same thing with your second cell How many cells do you have as a result of Meiosis? Compare the chromosomes of your gametes. What do you notice? How many cells do you have as a result of Meiosis? Compare the chromosomes of your gametes. What do you notice?