WEEK 5 CYTOGENICS DNA MUTATION Flashcards
What is cytogenetics?
- the examination/visualisation of chromosomes
- “study of inheritance by visualising the structure and function of chromosomes”
How many chromosomes do humans have?
- 46 chromosomes (23 pairs of homologues)
- 1 pair of sec chromosomes
Where is the centromere in a matacentric chromosome?
- In the MIDDLE
Where is the centromere in a acrocentric chromsome?
- Towards rthe end
Where is the centromere in a telocentric chromosome?
- At the end
Can you describe the chromosomes by centromere position and size?
- YES
- But this is NOT SUFFICIENT to distinguish them all
What is one method of identifying individual chromosomes in mammals?
- Giemsa stain
What is the process of G banding?
- Pre treat chromosomes on a slide with agent that differentially LOOSENS DNA-protein interaction
- It is then stained with Giemsa–> G bands (dark and pale)
What is the function of Colcemid?
- Mitotic poison that stops cells at mitosis
What are the proportions of Carnoy’s fixative that the Victoria Cancer Cytogenetics Service use?-
- 3 parts methanol and 1 part acetic acid
In which stage of mitosis do you want to catch cells to observe chromosomes clearly?
- Metaphase –> So you can karyotype
What is the order for the nomenclature of human karyotypes?
In order from: 1. Number of chromosomes in cell 2. Complement of sex chromosomes 3. Any abnormal chromosomes e.g. 46, XY normal male 46, XX, normal female 47, XY, +21 is male with extra copy of chr. 21 (Down syndrome)
What is the nomenclature of human chromosomes in order?
- p-short arm; q-long arm
- Each arm is subdivided into regions by landmark G-bands
- Each band (dark and pale is numbered WITHIN regions starting FROM THE CENTROMERE
e. g. 7q36–> chr.7, long arm q , region 3, G band 6
What is the most recent method for distinguishing between chromosomes?
- An extension of the FISH procedure
What is the method of chromosome painting?
- Make probe with MANY sequences from individual chromosome
- Isolate the chromosome cytologically OR obtain DNA from library of clones from that chromosome
- Label probe CHEMICALLY with fluorescent dye
- HYBRIDISE probe to chromosome spreads IN SITU
- Chromosome of one type will fluoresce at many different sites along its length –> “painted”
What are 2 examples of the wide range of reproductive modes found?
- Asexual organisms
- Some alternate b/w periods of sexual repro and lolng periods of asexual repro
What type of reproduction do most DIPLOID organisms have?
- Only SEXUAL REPRODUCTION
What does sexual reproduction require in terms of genetics?
- Sexual differentiation OR phenotypic dimorphism of the two sexes
What type of chromosomes (what is the fancy name) distinguish the two sexes?
- heteromorphic chromosomes
What are the basis of sex DETERMINATION?
- not just the genes on sex chromosomes, also normal chromosomes other than X and Y
What is the proportion of genetic sex determining systems?
- Populations of females 50% to males 50%
- One sex is heterogametic, the other is homogametic
Do non gametic systems have equal or unequal numbers of sexes?
- Unequal
Is the ratio of males to females 1?
- NO
- Varies in DIFFERENT countries e.g. males 1.20
What are 3 possible reasons as to why males are favored compared to females?
- May produce more Y bearing sperm than females
- Y bearing sperm might be more viable and motile
- Egg surface may be more receptive to Y bearing sperm
What did human karyotypes show that determines maleness?
- Presence of Y chromosome
Does the X or Y chromosome have more predicted genes?
- X chromosome
What are pseudo-autosomal regions in genes?
- Genes that are HOMOLOGOUS (shared) between the X and Y chromosomes (PAR1 and PAR2)
Are there any crossovers between the PARs?
- YES
- Always one crossover in the paired pseudoautosomal region at the end of the short arm (red)
What does the SRY gene stand for and where is it found?
- Sex Determining Region
- Found on Y chromosome (short arm-p)
How was SRY located?-
- Through rare and abnormal cases of XX males and XY females through rare crossovers BELOW the SRY gene at meiosis
Do individuals with 46, XX (male) or 46 XY (female) transmit the defect to the next generation?
- NO
- Because they are sterile
What is the sex determining gene and how was it done?
- SRY
- If inserted into transgenically normal XX female mice, they would turn male
Is the method for sex determination the same in Drosophila as is for humans?
- NO
What does the presence of the Y chromosome in Drosophila do?
- Only required for functional sperm
- Sex is determined by the ratio of autosomes to sex*
What is the phenomena of dosage compensation?
- Most genes on X chromosome not involved in sex differences so there is compensation for the differences in gene dosage for males and females (bc. females have two X chromosomes and males only one)
What is an example of dosage compensation?
- G6PD (glucose-6-Phosphate dehydrogenase) is in RBCs and gene is on X chromosome
- Shows very similar levels of it b/w males and females despite females having XX
How is dosage compensation achieved in mammals?
- RANDOM INACTIVATION of one of the two X chromosomes in each cell EARLY in female dev. (15-16 days of dev)
- The inactivated state is propagated to all progeny cells
- Most genes on the inactivated X chromosome are SILECNED
- Then silenced X becomes REACTIVATED for oogenesis