Ch. 3: The Chromosome Theory of Inheritance

Diakinesis

-Chromatids will thicken and shorten -At the end of prophase 1, the nuclear membrane breaks down and the spindle starts to form.

Chorionic villus sampling

-Larger samples are collected than in an amniocentesis, and can be performed earlier in pregnancy. (10-13 weeks after last period) -Slightly increases risk of miscarriage -Doctor will remove some chorionic villi from the placenta, which can then be analyzed for chromosomal defects.

Diplotene

-The synaptonemal complex dissolves -a tetrad of 4 chromatids become visible. -Crossover points are visible as chiasmata that hold nonsister chromatids together. -Meiotic arrest occurs at this point in many species.

Correlating chromosomes with Mendel's heredity units

1. Every cell has 2 copies of each type of chromosome, just like genes. 2. Chromosomes complement, like Mendel's genes, and is unchanged in it transfer from parents to offspring. 3. During meiosis, homologous chromosomes pair and then separate to different gametes, just like Mendel's traits. 4. Maternal and paternal chromosomes assort independently of other maternal and paternal chromosomes, just like independent assortment of alleles. 5. At fertilization, an egg's chromosomes fuse with a random sperms chromosomes, just as alleles do. 6. In all cells derived from fertilized egg, one-half of the chromosomes and one-half of the genes are maternal, the other half paternal.

Human chromosome

46 chromosomes, 22 matching *autosomes* and the sex chromosomes. Each pair differs in size, w/ 1 as the largest, 22 as the smallest (except for a few mixups)

Walter S. Sutton

A freakin' 'Murica man in the early 20th century that figured out chromosomes determine sex. -Looked at grasshopper sperm and saw that of the 24 chromosomes, only 22 matched in males. -He called the larger ones X, smaller ones Y. But females had all X.

Coenocyte

A plant cell with two or more nuclei (e.g. coconut milk is composed of these)

Sex determination in humans

All the boring stuff, but the key distinction is that: It is the presence of the Y chromosome that determines gender in humans, not the # of X chromosomes. e.g. A *klinefelter male (XXY)* will appear as a male and live a primarily normal life. Conversely, a *Turner female (X)* will appear female.

Syncytium

An animal cell with two or more nuclei (sim city hmm?)

Dyeing chromosomes

Cells in metaphase can be fixed and stained with special yes that highlight the centromeres -The dye also makes characteristic banding patterns, allowing us to id which chromosome it is -If the chromosomes match in size, shape, and banding pattern they are *homologous*, one would've been maternal, the other paternal. -Unrelated chromosomes are considered nonhomologous.

Crisscross inheritance

Characteristic of sex-related genes. -It is said to have occurred when a male inherits the trait of his mother, while the female inherits the trait of her father.

Leptotene

Chromosomes thicken and become visible, but the chromatids remain invisible. Centrosomes will start to move towards the poles.

Chromosomes

Discovered in the 1880s due to the discovery of handy dandy dyes. Literally translates to coloured bodies. -We could follow their movement during cell division, allowing us to differentiate between mitosis and meiosis Haploid cells are *n* Diploid cells are *2n* Drosophila have 4 unique chromosomes (so 8 total in diploid cells)

Amniocentesis

Doctor puts a needle through a woman's abdominal wall into the amniotic sac about 16 weeks after the woman's last period. -The doctor will then withdraw some amniotic fluid, to be analyzed for the DNA of the child to see if they'll have a genetic condition. -Fluid has *amniocytes* which can be grown on culture medium.

gene zw10

Encodes a component of the kinetochores that attach to chromosomes. -When it is defective disrupts mitotic chromosomal segregation. Chromosomes tend not to segregate properly, with more going to one cell than the other. This ultimately screws the organism.

Down syndrome

Genetic disorder due to having too many chromosomes (trisomy 21) -Have a characteristic wide skull that is flat at the back, large tongue, and learning disabilities from weird hippocampus development.

Zygotene

Homologous chromosomes enter synapsis (they zip together), and the synaptonemal complex forms (it is the zipper, which makes sure the chromosomes are properly aligned).

Sex determination in birds and butterflies

In these species, males have the matching sex of sex chromosomes (ZZ) and females are not matched (ZW). The gender w/ the 2 different sex chromosomes is termed the *heterogametic sex*, as it'll make 2 different kinds of gametes.

Sex determination in fruit flies

It is the ratio of X chromosomes to autosomes that determine sex for these guys. Females will be 1:1 (2 X chromosomes, two of each autosome) Males will be 1:2 (1 X, 2 autosomes) 2:3 makes an intersex fly While Y doesn't determine sex, it is needed for male fertility, so an X male will be sterile.

Metaphase I

Only important distinction is that kinetochores of sister chromatids fuse such that each chromosome only has a single functional pair, with maternal facing one pole with its kinetochore and paternal facing the other pole with its own kinetochore. -This ensures segregation of maternal and paternal chromosomes between gametes

Regulatory checkpoints

Points during the cell cycle in which the cell assesses the results of the previous steps. G1/S phase: Is the cell big enough? Did we get the make a baby signal hormone? Alright duplicate them chromosomes and centrosomes. G2/Mitosis: Have the chromosomes been properly duplicated? Good stuff, no split 'choself. Metaphase/anaphase: Have all of the chromosomes been aligned on the metaphase plate? Then snip that thar cohesin.

Oogenesis

Produces a large, nutrient rich ovum at the end. 1. *Oogonia* (diploid germ cells in the ovary) divide by mitosis to make *primary oocytes* 2. *Primary oocytes* undergo meiosis, the first division is asymmetric (one gets more cytoplasm than the other), with the larger referred to as the *secondary oocyte* and the smaller is the *first polar body*. 3. the *secondary oocyte* undergoes meiosis 2, again asymmetric, and the larger cell makes a large haploid *ovum* while the smaller cell is the *second polar body*. The first often will not undergo meiosis 2. This process begins in the fetus, and arrests in diplotene of meiosis I, at about age 12 until menopause most women release one primary oocyte each month. The remainder desintegrate during menopause. At ovulation, an oocyte completes meiosis I and goes as far as metaphase II, and will continue past that point if it becomes fertilized.

Primary nondisjunction

Refers to the first nondisjunction event. Secondary nondisjunction has occurred if primary has already occurred earlier in that genome If they do split, XY and X gametes are far more prominent than XX and Y gametes

Germ cells

Set aside early in development for the production gametes via meiosis in humans. (plants don't make them until floral development)

Spermatogenesis

Starts in the testes 1. *Spermatogonia* divide mitotically to make *primary spermatocytes* 2. *primary spermatocytes* undergo symmetrical meiosis 1 to produce two *secondary spermatocytes* 3. Each *secondary spermatocyte* undergoes symmetrical meiosis II to produce 4 total *spermatids* 4. *spermatids* mature by developing the whiplike tail and making a head, to become a functional sperm containing 22 autosomes and an X or Y sex chromosome. The timing of this one changes a lot, it begins only at puberty, then goes on throughout the rest of a man's life: takes about 48-60 total days 16-20 for meiosis I 16-20 for meiosis II 16-20 for the maturation of spermatids.

Pachytene

Synapsis is complete, and crossing-over (genetic exchange between nonsister chromatids of a homologous pair) occurs. -Each synapsed chromosome pair is a bivalent/tetrad Sex chromosomes has a pseudoautosomal region (PAR) in which synapsis can occur.

Metacentric chromosomes

The centromere of these chromosomes are near the middle.

Acrocentric chromosomes

The centromere of these chromosomes are towards one end.

3 types of microtubules that form the mitotic spindle

There are 3 kinds, and all of them originate from the centrosome: 1. Kinetochore microtubules: Extend between a centrosome and a kinetochore of a chromatid. (centromeric fibres) 2. Polar microtubules: Microtubules from each centrosome that are directed toward the middle of the cell, which interdigitate near the cell's equator. 3. Astral microtubules: Short microtubules that extend from the centrosome towards the cell periphery. It is unstable for both kinetochores to be attached to microtubules from the same pole, and one will keep detaching until another microtubule binds oh so right.

Nondisjunction

When homologues of a chromosome fail to segregate during meiosis I