6.1 Chromosomes & Meiosis

You have body cells and gametes. Somatic cells – body cells make up body tissues and organs DNA from body cells DOES NOT pass to offspring undergo mitosis Gametes – sex cells developed from germ cells in reproductive organs ova (eggs) or spermatozoa (sperm) DNA from gametes CAN pass to offspring undergo meiosis

Chromosomes Each species has a unique number of chromosomes. number of chromosomes = complexity Humans Somatic cells contain a set of 46 (23 pairs). These cells are genetically identical. Cells within an organism differ from each other because genes are expressed not because they have different genes.

Your cells have autosomes and sex chromosomes. Humans have 46 (23 pairs) of chromosomes. 23 from mother and 23 from father Each pair is a homologous pair (same structure). Homologous chromosomes - copies of same genes

Fig 6.1

Autosomes & Sex chromosomes Chromosome pair 1 -22 Contain genes for characteristics not related to gender 23rd chromosome pair How gender is determined. Humans have a XY system. XX = female XY = male X and Y are not homologous! X is larger and contains many genes, some unrelated to sexual characteristics. Y is smaller and contains genes that develop male traits (testies)

Body cells are diploid Gametes are haploid Quick Review Time: Sexual Reproduction - the fusion of two gametes; offspring are a genetic mix Fertilization – fusion of egg & sperm Nuclei of egg and sperm cell fuse to form one nucleus. New nucleus must have the correct number of chromosomes for a healthy new organism to form.

Body cells are diploid Gametes are haploid Somatic (body) cells 2n = 46 chromosomes Cells have 2 copies of each chromosome (mother/father) Gametes n = 23 chromosomes Cells have 1 copy of each chromosome Each egg/sperm has 22 autosomes and 1 sex chromosome Vocabulary p170

6.2 Cell Division Mitosis Vs. Meiosis

Mitosis Form of nuclear division in somatic (body) cells. produces daughter cells that are genetically identical to parent cell. DNA is copied once; divided once. Parent and daughter cells are diploid. Mitosis is used for: Development Repair Growth Reproduction in asexual organisms eukaryotes

Meiosis Form of nuclear division that divides a diploid cell into a haploid cell. needed for sexual reproduction Reduction division – it reduces chromosome number by half DNA is copied once; but divided twice makes genetically unique haploid cells these haploid cells then undergo more development in ovaries or testes to form mature gametes

Process of Meiosis Form of nuclear division that divides a diploid cell into a haploid cell. Process involves 2 rounds of cell division: Meiosis I and Meiosis II helps create genetic diversity

Homologous chromosomes Sister chromatids two separate chromosomes – mother/father similar in length and carry same genes not copies! divided in meiosis I two chromatids (half of duplicated chromosome) - divided by meiosis II

Interphase G1 phase: S phase: G2 phase: cell increases in mass in preparation for cell division. S phase: DNA is synthesized. G2 phase: cell synthesizes proteins and continues to increase in size. In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus.

Prophase I DNA condenses and chromosomes become visible Homologous chromosomes are attracted to each other and pair up Due to similar chromosome structure and allele alignment, crossing over occurs Spindle fibers made from microtubules form at the poles of the parent cell

Metaphase I Homologous chromosomes align along the cell’s equator randomly The nuclear membrane disintegrates

Anaphase I Spindle fibers from the pole separate the homologues and pull them towards the poles Telophase I Spindles/fibers disintegrate Chromosomes uncoil and nuclear membrane reforms Cytokinesis occurs afterwards, resulting in two haploid cells.

Prophase II DNA condenses into visible chromosomes again New spindle fibers are produced Metaphase II Nuclear membrane disintegrates Chromosomes align randomly along the equator (random orientation) Spindle fibers attach at centromeres

Anaphase II Sister chromatids split at the centromere and are pulled to opposite poles by the spindle fibers New membranes (animals) or cell plates (plants) form between new cells Telophase II Chromosomes unwind New nuclear membranes form around the four haploid cells before cytokinesis

During prophase I, homologous chromosomes are able to swap genes through a process known as crossing over. When this occurs, a chiasma forms between the chromosomes. It is where the two chromosomes connect and exchange genetic material.

When chromosomes form chiasmata, they are oriented in the same direction with their loci mostly aligned. This means that alleles on each chromosome are lined up. When DNA is exchanged and ‘crossed over,’ alleles for the same genes are swapped. The exchange is NOT random. A a

Chromosomes that are a result of crossing over have both paternal and maternal DNA. Note that both chromosomes have the same loci and orientation, but not always alleles. This crossing over during prophase I combined with random orientation during metaphse I effectively results in infinite genetic variability (223 without crossing over!) 10.1.3 - Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.

Crossing over can occur multiple times between homologous chromosomes with either one or both chromatids. The possibilities are endless! 10.1.3 - Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.

Mendel’s Law of Independent Assortment states that during gamete formation allele pairs separate independently from other pairs. For example, the alleles that determine hair color are not bound to the alleles for skin color. 10.1.4 - State Mendel’s law of independent assortment.

Random orientation during metaphase I and random crossing over during prophase I support explain why alleles are able to segregate independently of each other. 10.1.5 - Explain the relationship between Mendel’s law of independent assortment and meiosis.

10.1.5 - Explain the relationship between Mendel’s law of independent assortment and meiosis.