genet 302 lec 4 & 5
Why do metaphase cells show chromosomes best?
Because chromosomes are fully condensed and separated, making them easiest to see and count.
How are chromosomes prepared for microscopy?
Cells are burst (dropped) onto a slide so nuclei break open and chromosomes spread out, then detection reagents (stains) are added.
What are coloured DNA stains?
Chemicals with a natural affinity for all DNA that have a visible colour (seen with regular light microscopy).
What does Orcein stain?
All DNA, making mitotic chromosomes visible as dark-coloured structures.
What is a fluorescent DNA stain?
A molecule that binds all DNA and glows a specific colour under fluorescent light.
What does DAPI stain?
All DNA, and it fluoresces blue, allowing all chromosomes to be visualized.
What is a hybridization probe?
A piece of single-stranded DNA (ssDNA) attached to a fluorescent dye that binds a specific DNA sequence by base pairing
What does FISH stand for?
Fluorescence In Situ Hybridization — using fluorescent DNA probes to detect specific DNA sequences in chromosomes
What DNA sequence was targeted in the example?
Telomere DNA (TTAGGG)n, which was labeled red by a hybridization probe.
What do fluorescent antibodies detect?
Specific chromatin proteins, not DNA.
How do antibody probes work?
An antibody that binds a target chromatin protein is covalently linked to a fluorescent dye, lighting up where that protein is located.
What was detected in the example with polytene chromosomes?
MSL3 chromatin proteins using fluorescent antibodies.
How many chromosomes do fruit flies have?
Drosophila have 4 chromosome pairs:
X/Y (sex chromosomes), 2, 3, and 4.
What do the colours mean in Drosophila chromosome diagrams?
Blue = heterochromatin (satellite DNA)
• Yellow = euchromatin (gene-rich DNA)
• C = centromere
• NO = nucleolar organizer (rRNA genes)
What is special about heterochromatin?
It contains repetitive satellite DNA, is gene-poor, and clusters together in polytene chromosomes.
What type of chromosome is the Drosophila X chromosome?
Telocentric — the centromere is near one end of the chromosome.
Where do polytene chromosomes come from?
Salivary gland cells of Drosophila larvae.
What causes polytene chromosomes to form?
Repeated DNA replication without mitosis, while sister chromatids remain stuck together.
What three things happen to make a polytene chromosome?
Sister chromatids stick together
Homologous chromosomes stick together
All heterochromatin sticks together → chromocentre
What is the chromocentre?
The central mass where all heterochromatin from all chromosomes sticks together.
What forms the arms of a polytene chromosome?
Euchromatin, which contains most of the genes.
Why does one polytene chromosome represent eight chromosomes?
It contains two homologs × two sister chromatids × replicated many times bundled into one structure.
Why are polytene chromosomes enormous?
They contain hundreds to thousands of DNA copies aligned side-by-side, making them visible under a light microscope.
Do male and female larvae have different numbers of polytene arms?
No — males have a Y chromosome, but it does not form polytene arms, so both sexes have the same number of arms.
What causes the dark and light banding pattern?
Dark bands = compact chromatin
• Light interbands = open, transcriptionally active chromatin
What stain is used to enhance polytene bands?
Orcein, a coloured DNA stain that makes bands darker.
What is a recombination map?
A map showing gene order based on crossing-over frequency, invented by Alfred Sturtevant.
Where is the white (w) gene on the recombination map?
At 1–1.5 map units on the X chromosome.
What is a cytogenetic map?
A map based on visible chromosome bands, created by Calvin Bridges using polytene chromosomes.
Where is the white gene on the cytogenetic map?
At 3B6 on the X chromosome.
What is a DNA sequence map?
A map based on exact base-pair positions in the genome (completed in 2000).
Where does the white gene start in the DNA sequence map?
At X: 2,684,632 base pairs.