DNA and the Genetic Basis of Life: Another Great Divide All About You Science Teacher Education Course

Bell Ringer How many chromosomes are in a human somatic (body) cell?

What is sexual reproduction? Sexual reproduction—the process in which genetic material from two parents combines and produces offspring that is genetically different from each parent—is how humans create new life. Children resemble their parents because they inherit half their genetic material from their mother and the other half from their father. In order to ensure the genetic diversity of offspring while keeping the amount of genetic material the same, sexual reproduction depends on meiosis, which is the cell division process that creates specialized sex cells. Refer to Figure 1. Each parent of a sexually reproducing species contributes a unique sex cell (gamete). Males produce sperm in their testes and females produce eggs in their ovaries. Each sex cell contains a haploid number of chromosomes, or half the number of chromosomes. When a sperm cell successfully fertilizes an egg cell, their nuclei fuse. They produce a diploid somatic cell called a zygote. A zygote is a cell that contains the full number of chromosomes. For humans, the haploid number is n=23, and the diploid number is 2n=46 (or 23 pairs). This zygote inherits half of each pair of chromosomes from the mother, and inherits the other half of each pair of chromosomes from the father. The zygote will undergo countless rounds of cell division, which includes mitosis and cytokinesis, to become a fully developed human being.

Interphase Precedes meiosis The diploid cell can grow by obtaining energy from food, transporting materials into and out of the cell, and preparing for cell division Originally, there was only one copy of the purple DNA and one copy of the green DNA but now they are duplicated In early interphase, the nucleolus is still visible In late interphase, the nucleolus disappears and DNA condenses into chromatin The nuclear membrane is still intact Centrioles may also be visible at this time Cytoplasm Cell membrane Chromatin Centrosome (centrioles) Nuclear membrane Nucleus

Meiosis I: Prophase I Duplicating homologous chromosomes Chromatin is now condensed into chromosomes First, we see a pair of homologous chromosomes of a diploid parent cell The green chromosome was inherited from the mother and the purple chromosome was inherited from the father Each pair of homologous chromosomes in the diploid cell is duplicated and the copies are called sister chromatids and they are attached by a protein disk called a centromere The resulting pair of duplicated homologous chromosomes is called a tetrad (there are four)

Meiosis I: Prophase I (continued) Crossing over Each pair of duplicated homologous chromosomes (non-sister chromatids) exchanges genetic material The homologous chromosomes line up so that they only exchange parts of the same genes This ensures that at the end of meiosis, each sex cell will contain a unique genetic make-up and helps to explains why siblings, with the exception of identical twins, do not look exactly alike Tetrad Centromere Sister chromatids

Meiosis I: Prophase I (continued) The chromosomes are found in between the two poles of the cell The centrioles are now at either poles of the cell Spindle microtubules extend from the centrioles towards the center of the cell Cytoplasm Cell membrane Spindle microtubules Chromosomes Centrosome (centrioles)

Centrosome (centrioles) Meiosis I: Metaphase I Tetrads (duplicated homologous chromosomes) line up along the middle of the cell Chromosomes face opposite poles of the cell Spindle microtubules extend towards the center of the cell and some attach to the centromeres while other spindle microtubules hang into the cytoplasm Cell membrane Cytoplasm Chromosomes Spindle microtubules Centrosome (centrioles)

Centrosome (centrioles) Meiosis I: Anaphase I The spindle microtubules attached to the centromere of each homologous chromosome separate them to opposite ends of the cell (note that the sister chromatids stay together) Cell membrane Chromosome Cytoplasm Chromosome Spindle microtubules Centrosome (centrioles)

Meiosis I: Telophase I and Cytokinesis Each half of the cell contains one copy of the chromosomes, thus setting up the correct environment for the cells to become haploid Spindle microtubules disintegrate and cytokinesis begins Cell membrane and cytoplasm pinch inward until two new haploid cells are produced Afterwards, meiosis II can begin Chromosome Cell membrane Daughter cells Cleavage furrow Cytoplasm Chromosome Centrosome (Centrioles)

Meiosis II: Prophase II Spindle microtubules begin to form at the poles of the two haploid cells Chromosomes, consisting of two sister chromatids each, migrate towards the center of the cell (recall that due to crossing over, the sister chromatids are no longer genetically identical to one another) Cytoplasm Centrosome (Centrioles) Cell membrane Spindle microtubule Chromosome

Meiosis II: Metaphase II Chromosomes line up in the middle of the cell Each sister chromatid faces the opposite end of the cell Spindle microtubules extend towards the center of the cell and some attach to the centromeres, while others hang into the cytoplasm Chromosome Cell membrane Centrosome (Centrioles) Cytoplasm Spindle microtubules

Meiosis II: Anaphase II The spindle microtubules attach to the centromere of each sister chromatid and separate them to opposite ends of the cell Sister chromatid Sister chromatid Cell membrane Centrosome (Centrioles) Cytoplasm Spindle microtubule

Meiosis II: Telophase II and Cytokinesis Sister chromatids reach the opposite end of each cell and nuclear membranes form around them Spindle microtubules disintegrate Cell membrane and cytoplasm pinches inward to create new cells The result is four genetically different haploid cells that can be sperm or egg cells Cytoplasm Cell membrane Nucleus Nuclear membrane Chromatin

Karyotype Centromere Sister chromatids Pair of homologous chromosomes 5 Sex chromosomes 15

Down Syndrome Trisomy 21, called Down syndrome, produces a characteristic set of symptoms, which include: mental retardation, characteristic facial features, short stature, heart defects, susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and shortened life span. The incidence increases with the age of the mother.

Figure 8.19A Figure 8.19A A karyotype showing trisomy 21, and an individual with Down syndrome Trisomy 21 17

Accidents Occur in Meiosis Nondisjunction is when chromosomes or chromatids do not separate normally during meiosis. This can happen during meiosis I, if both members of a homologous pair go to one pole or meiosis II if both sister chromatids go to one pole. Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes.

Multicellular diploid adults (2n  46) Diploid stage (2n) Figure 8.12A Haploid gametes (n  23) n Egg cell n Sperm cell Meiosis Fertilization Ovary Testis Figure 8.12A The human life cycle Diploid zygote (2n  46) 2n Key Mitosis Haploid stage (n) Multicellular diploid adults (2n  46) Diploid stage (2n) 19

MEIOSIS I Nondisjunction MEIOSIS II Normal meiosis II Gametes Figure 8.20A_s3 MEIOSIS I Nondisjunction MEIOSIS II Normal meiosis II Figure 8.20A_s3 Nondisjunction in meiosis I (step 3) Gametes Number of chromosomes n  1 n  1 n  1 n  1 Abnormal gametes 20

MEIOSIS I Normal meiosis I MEIOSIS II Nondisjunction n  1 n  1 n n Figure 8.20B_s3 MEIOSIS I Normal meiosis I MEIOSIS II Nondisjunction Figure 8.20B_s3 Nondisjunction in meiosis II (step 3) n  1 n  1 n n Abnormal gametes Normal gametes 21

Mistakes with Sex Chromosomes Sex chromosome abnormalities tend to be less severe, perhaps because of The Y-chromosome being very small and carrying very little genetic information or Extra X-chromosomes becoming inactivated and the organism operating with one functioning one X- chromosome

Errors in Cell Division Can Make New Species Errors in mitosis or meiosis may produce polyploid species, with more than two chromosome sets. The formation of polyploid species is widely observed in many plant species but less frequently found in animals. Gray Tree Frog Tetraploid organism (4n) Scientists still do not understand how polyploidy can cause organisms to so different from their diploid ancestors

Alterations to a Chromosome’s Structure Can Cause Birth Defects and Cancer Deletion- the loss of a chromosome segment Duplication- the repeat of a chromosome segment, Inversion- the reversal of a chromosome segment Translocation- the attachment of a segment to a nonhomologous chromosome that can be reciprocal

Reciprocal translocation Figure 8.23A Deletion Inversion Duplication Reciprocal translocation Homologous chromosomes Nonhomologous chromosomes Figure 8.23A Alterations of chromosome structure 25