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 the number and visual appearance of the chromosomes in the cell nuclei Autosomes & Sex chromosomes Autosomes - Chromosome pair 1 -22 Sex chromosomes
- 23rd chromosome pair - Contain genes for characteristics not related to gender - 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 Diploid - Somatic (body) cells Haploid - Gametes
- 2n = 46 chromosomes - n = 23 chromosomes - Cells have 2 copies of each chromosome (mother/father) - 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 - two chromatids (half of duplicated chromosome) - similar in length and carry same genes
- divided by meiosis II - not copies! - divided in meiosis I Interphase G1 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 Metaphase I Homologous chromosomes align along
the cells 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 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
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 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. A A A
a A a a a
a A a A When DNA is exchanged and crossed over, alleles for the same genes are swapped. The exchange is NOT random.
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 metaphase I effectively results in infinite genetic variability (223 without crossing over!)
Crossing over can occur multiple times between homologous chromosomes with either one or both chromatids. The possibilities are endless! Mendels 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.
Random orientation during metaphase I and random crossing over during prophase I support explain why alleles are able to segregate independently of each other.
