genet 302 lec 3
What does n mean in genetics?
n = number of chromosomes in a gamete (haploid set). In humans, n = 23.
What does c mean in genetics?
c = amount of DNA in a gamete (one full genome’s worth of DNA), regardless of chromosome structure.
What does diploid mean?
Diploid cells have two copies of each chromosome (2n) — one from each parent. Sex chromosomes are an exception (XX or XY).
What is the ploidy of humans?
Humans are diploid (2n = 46); sperm and eggs are haploid (n = 23)
Are fruit flies diploid?
Yes. Drosophila have diploid somatic cells, just like humans.
What is euploid vs aneuploid?
Euploid = correct number of chromosomes (e.g., 2n = 8 in Drosophila)
Aneuploid = abnormal number (missing or extra chromosomes)
What is polyploidy?
Having more than two copies of each chromosome.
Examples:
3n = triploid
4n = tetraploid
6n = hexaploid
8n = octoploid
Why are bananas seedless?
Bananas are triploid (3n) → their chromosomes can’t pair properly in meiosis, so they are sterile.
Why is durum wheat polyploid?
Pasta wheat is tetraploid (4n) — every cell has four copies of each chromosome.
What is germline polyploidy?
When all cells in the organism are polyploid (e.g., bananas, wheat).
What is somatic polyploidy?
When some cells in an otherwise diploid organism become polyploid.
What are the advantages of polyploid cells?
* Larger
• Can make/export more proteins
• More metabolic capacity
What is the major disadvantage of polyploid cells?
They usually cannot divide, so they can’t reproduce.
What are the phases of a normal cell cycle?
G1 → S → G2 → M
M phase = prophase → metaphase → anaphase → telophase → cytokinesis
What is cell fusion?
When two or more diploid cells merge, combining their nuclei → one polyploid cell.
Which human cells use cell fusion?
Skeletal muscle cells (myoblasts fuse into long muscle fibers).
Why do muscle cells fuse?
To create giant multinucleate cells that can produce huge amounts of contractile proteins.
What happens if a cell skips cytokinesis?
Mitosis happens, but the cell does not split, producing a multinucleated polyploid cell.
Which organism uses this in embryos?
Drosophila embryos.
Why do fly embryos skip cytokinesis early on?
To rapidly create thousands of nuclei inside one big cell before forming cell membranes.
What is a syncytium?
A single large cell with many nuclei, created by skipping cytokinesis. (Early fly embryos are syncytial.)
What happens if telophase and cytokinesis are skipped?
Chromosomes separate but rejoin into one nucleus, making a single giant polyploid nucleus.
Which human cells use this mechanism?
Megakaryocytes (platelet-producing cells).
Why do megakaryocytes become polyploid?
To become huge factories for making and exporting platelets.
What happens to red blood cells?
They expel their nucleus during maturation, becoming smaller and more flexible.
What happens if mitosis is skipped?
DNA keeps replicating (S phase repeats) → nucleus becomes extremely polyploid.
Which cells do this in fruit flies?
Larval organ cells (salivary glands, gut, etc.).
Why are larval cells polyploid?
They need to make huge amounts of proteins (e.g., sticky salivary glue) but do not divide.
What are imaginal discs?
Diploid cells inside larvae that will become adult tissues (they must still divide).
Where is somatic polyploidy found in humans?
Skeletal muscle
• Megakaryocytes
• Cancer cells
• Placenta
• Liver
Which mechanisms create these?
Muscle → cell fusion
Megakaryocytes → skip telophase & cytokinesis
RBCs → nucleus expelled
Liver & placenta → endocycles (skip mitosis)
What does “salivary glue proteins made by sgs genes” mean in Drosophila larvae?
sgs (salivary gland secretion) genes are highly amplified in polyploid salivary gland cells, allowing the larva to make huge amounts of sticky glue proteins that attach it to a surface during pupation (metamorphosis).