Meiosis Chapter 10 Biology In Focus AP Biology 2014 Ms. Eggers

Genetics Heredity/inheritance: terms that describe the transmission of traits from one generation to the next Heredity/inheritance: terms that describe the transmission of traits from one generation to the next Variation: traits may be transferred but some level of variation often exists in those traits between generations Variation: traits may be transferred but some level of variation often exists in those traits between generations Genetics: the study of heredity and hereditary variation Genetics: the study of heredity and hereditary variation

Heredity has its basis in our DNA Our traits result from the action of proteins coded for in our DNA Our traits result from the action of proteins coded for in our DNA The unit of DNA that is used to produce a given protein is a gene The unit of DNA that is used to produce a given protein is a gene The genes that determine our traits are delivered to us via gametes, or sex cells, which come from our mother and our father The genes that determine our traits are delivered to us via gametes, or sex cells, which come from our mother and our father

Diploid somatic cells make up most of the cells in our bodies The cells of our body, or somatic cells, are made up of chromosome from our mother and chromosomes from our father The cells of our body, or somatic cells, are made up of chromosome from our mother and chromosomes from our father This means that every chromosome is paired This means that every chromosome is paired The term for a cell with paired chromosomes is diploid The term for a cell with paired chromosomes is diploid We have 46 pairs of chromosomes – 23 came from our mother and a matching set of 23 came from our father We have 46 pairs of chromosomes – 23 came from our mother and a matching set of 23 came from our father

Karyotype: a picture of a somatic cell’s chromosomes arrested in metaphase

Homologous chromosomes A pair of chromosomes can be identified in a karyotype by having the same length, centromere position, and staining pattern A pair of chromosomes can be identified in a karyotype by having the same length, centromere position, and staining pattern Both chromosomes of the pair carry the genes for the same traits Both chromosomes of the pair carry the genes for the same traits

One pair is different… We have 22 pairs of homologous chromosomes which are identical – these are referred to as autosomes We have 22 pairs of homologous chromosomes which are identical – these are referred to as autosomes The 23 rd pair of chromosomes are known as the sex chromosomes because they are responsible for sex determination The 23 rd pair of chromosomes are known as the sex chromosomes because they are responsible for sex determination They are the X and Y chromosomes They are the X and Y chromosomes If an embryo receives two X chromosomes, it will develop as a female If an embryo receives two X chromosomes, it will develop as a female Receiving an X and a Y will result in the development of a male Receiving an X and a Y will result in the development of a male

Asexual reproduction Some organisms can produce copies of themselves by mitosis Some organisms can produce copies of themselves by mitosis In this case, the offspring ends up being an EXACT genetic copy of its parent. In this case, the offspring ends up being an EXACT genetic copy of its parent. This offspring is called a clone This offspring is called a clone We talked about binary fission in bacteria – this is a form of asexual reproduction We talked about binary fission in bacteria – this is a form of asexual reproduction An organism called the hydra can reproduce by budding, in which it grows a mini hydra on its body which then detaches An organism called the hydra can reproduce by budding, in which it grows a mini hydra on its body which then detaches

A budding hydra

Sexual reproduction Two parents give rise to offspring that have a unique combination of genes inherited from the two parents Two parents give rise to offspring that have a unique combination of genes inherited from the two parents

Why bother with sexual reproduction? Mitosis produces two genetically identical cells – no variation! Mitosis produces two genetically identical cells – no variation! Sexual reproduction allows for new gene combinations from the two parents Sexual reproduction allows for new gene combinations from the two parents New gene combinations might make an individual more fit and able to survive. This is the basis for evolution! New gene combinations might make an individual more fit and able to survive. This is the basis for evolution!

So why do we need a special kind of cell division (called meiosis) to make cells for sexual reproduction? If each parent gave ALL of their chromosomes then their offspring would have 92 chromosomes. The next generation would have 184, the next 368, and so on… If each parent gave ALL of their chromosomes then their offspring would have 92 chromosomes. The next generation would have 184, the next 368, and so on… The process of meiosis makes sperm and egg cells (gametes) and reduces the number of chromosomes to ½ - the cells produced are haploid. The process of meiosis makes sperm and egg cells (gametes) and reduces the number of chromosomes to ½ - the cells produced are haploid.

Sexual life cycles - variations

Meiosis occurs in 2 stages In meiosis I, homologous (matching) chromosomes pair up at the metaphase plate and then are split into 2 daughter cells. In meiosis I, homologous (matching) chromosomes pair up at the metaphase plate and then are split into 2 daughter cells. In meiosis II, the chromosomes now line up single-file along the metaphase plate, just like in mitosis. The chromatids are pulled apart at the centromere and split into 2 daughter cells. In meiosis II, the chromosomes now line up single-file along the metaphase plate, just like in mitosis. The chromatids are pulled apart at the centromere and split into 2 daughter cells. In the end you have 4 haploid cells. In the end you have 4 haploid cells.

Where does meiosis occur? In humans it occurs in the testes and ovaries (gonads) In humans it occurs in the testes and ovaries (gonads) Sperm are produced during spermatogenesis and eggs are produced during oogenesis. Sperm are produced during spermatogenesis and eggs are produced during oogenesis.

Prophase I Many of the events of mitotic prophase occur here – nuclear membrane dissolves, chromosomes condense, centrioles divide. Many of the events of mitotic prophase occur here – nuclear membrane dissolves, chromosomes condense, centrioles divide. In addition, homologous chromosomes pair up and become physically connected by a synaptonemal complex In addition, homologous chromosomes pair up and become physically connected by a synaptonemal complex This state is called synapsis and during this, a very important event called crossing over occurs This state is called synapsis and during this, a very important event called crossing over occurs

Where crossing over occurs, chiasmata can be observed

Genes come in different varieties – called alleles. Homologous chromosomes can have different alleles.

Crossing over is trading homologous segments of DNA – or swapping alleles.

Metaphase I Homologous chromosomes line up on the metaphase plate. Homologous chromosomes line up on the metaphase plate.

Anaphase I During anaphase I, the two homologous chromosomes move to separate ends of the dividing cell. During anaphase I, the two homologous chromosomes move to separate ends of the dividing cell. Unlike mitosis, sister chromatids are NOT separated at the centromere at this time. Unlike mitosis, sister chromatids are NOT separated at the centromere at this time.

Telophase I Same events as telophase of mitosis… Same events as telophase of mitosis… Let’s look at our chromosomes though. Now, our homologous pairs have been split up. There’s only one of each of the 23 pairs of chromosomes – although each chromosome has been duplicated and IS still stuck to its sister chromatid. Let’s look at our chromosomes though. Now, our homologous pairs have been split up. There’s only one of each of the 23 pairs of chromosomes – although each chromosome has been duplicated and IS still stuck to its sister chromatid. At the end of meiosis I we have two HAPLOID daughter cells At the end of meiosis I we have two HAPLOID daughter cells

Prophase II From meiosis I, we have the 2 daughter cells undergoing the second half of meiosis. From meiosis I, we have the 2 daughter cells undergoing the second half of meiosis. A replay of prophase during mitosis… A replay of prophase during mitosis…

Metaphase II Yup. Just like mitosis – the chromosomes line up single-file on the metaphase plate. Yup. Just like mitosis – the chromosomes line up single-file on the metaphase plate.

Anaphase II Just like mitosis, the sister chromatids are pulled apart at their centromere and are pulled to either pole of the dividing cells by the spindle fibers. Just like mitosis, the sister chromatids are pulled apart at their centromere and are pulled to either pole of the dividing cells by the spindle fibers. BUT, because of crossing over in meiosis I, the two sister chromatids are NOT identical BUT, because of crossing over in meiosis I, the two sister chromatids are NOT identical

Telophase II Now the cells complete division, undergoing cytokinesis. Now the cells complete division, undergoing cytokinesis. The result is 4 haploid cells  homologous chromosomes are no longer paired (meiosis I) and now the sister chromatids have been separated into individual chromosomes (meiosis II) The result is 4 haploid cells  homologous chromosomes are no longer paired (meiosis I) and now the sister chromatids have been separated into individual chromosomes (meiosis II)

Now here ’ s why you don ’ t look exactly like your mum or your dad When those homologous pairs of chromosomes pair up during prophase I of meiosis, they can be in either orientation. When those homologous pairs of chromosomes pair up during prophase I of meiosis, they can be in either orientation. This is called independent assortment This is called independent assortment

What does this mean?

Math… On the last slide, you saw that from 2 pairs of homologous chromosomes, we got 4 possible combinations in the gametes On the last slide, you saw that from 2 pairs of homologous chromosomes, we got 4 possible combinations in the gametes For humans, we can make 2 23, or 8.4 million different gametes from independent assortment alone. For humans, we can make 2 23, or 8.4 million different gametes from independent assortment alone. AND, since fertilization is random, there are now 2 23 x 2 23, or about 70 trillion possible combinations AND, since fertilization is random, there are now 2 23 x 2 23, or about 70 trillion possible combinations

And… crossing over - Crossing over produces recombinant chromosomes Crossing over produces recombinant chromosomes

Genetic diversity! In addition to this independent assortment of chromosomes, homologous pairs can mix-n-match through crossing- over. In addition to this independent assortment of chromosomes, homologous pairs can mix-n-match through crossing- over. These two types of genetic recombination assure that LOTS of possible gene combinations can be made by sexual reproduction. These two types of genetic recombination assure that LOTS of possible gene combinations can be made by sexual reproduction. Genetic diversity is what helps to guarantee that at least some of the offspring will live long enough to pass on their genes – this is the stuff of evolution! Genetic diversity is what helps to guarantee that at least some of the offspring will live long enough to pass on their genes – this is the stuff of evolution!

However, genetic recombination can go wrong and problems arise. There can be… Nondisjunction of chromosomes during meiosis I or II Nondisjunction of chromosomes during meiosis I or II Chromosomal abnormalities during crossing-over (in meiosis) Chromosomal abnormalities during crossing-over (in meiosis)

Nondisjunction Things can go wrong during meiosis I or meiosis II Things can go wrong during meiosis I or meiosis II In both cases, one daughter cell ends up with too many copies of a chromosome and the other with too few. In both cases, one daughter cell ends up with too many copies of a chromosome and the other with too few.

We can check for non- disjunction events by using karyotype analysis You can visually identify chromosomes that are absent, or too numerous, as well as major chromosomal abnormalities (like missing pieces). You can visually identify chromosomes that are absent, or too numerous, as well as major chromosomal abnormalities (like missing pieces).

You can get fetal cells from amniocentesis or chorionic villus sampling

An example of a disorder that results from nondisjunction is Trisomy 21 She has 3 copies of chromsome #21

Other examples of nondisjunction Down’s Syndrome (trisomy in chromosome 21) Down’s Syndrome (trisomy in chromosome 21) Turner Syndrome (XO) Turner Syndrome (XO) Klinefelter Syndrome (XXY) Klinefelter Syndrome (XXY) Metafemales (XXX) Metafemales (XXX) XYY Males XYY Males OY Males (lethal) OY Males (lethal)

What other problem can arise during meiosis? Chromosomal abnormalities can arise during crossing-over. Chromosomal abnormalities can arise during crossing-over. Deletion Deletion Duplication Duplication Inversion Inversion Reciprocal translocation Reciprocal translocation

A translocation: Karyotype name 46,XY t(9;22) aka Philadelphia chromosome A translocation of material between chromosome 9 and 22 is found in tumor cells in chronic myeloid leukemia (CML) A translocation of material between chromosome 9 and 22 is found in tumor cells in chronic myeloid leukemia (CML) This translocation causes CML cancer because part of a gene is moved from its normal location on chromosome 9 to a new location on chromosome 22. This translocation results in a mutated gene. This translocation causes CML cancer because part of a gene is moved from its normal location on chromosome 9 to a new location on chromosome 22. This translocation results in a mutated gene. The action of the mutant gene product leads to cancerous growth of the cells whose normal job is to produce blood cells for the body. The action of the mutant gene product leads to cancerous growth of the cells whose normal job is to produce blood cells for the body.

46,XY t(9;22) called Philadelphia chromosome