Topic 11 Flashcards
Describe the structure of a chromatid and the possible positions of the centromere
p arm and q arm p arm tends to be shorter and the one at the top metacentric p arms and q arms same length submetacentric p arms shorter than q arms acrocentric v short p arm telocentric no p arm (none in human body)
What are telomeres?
repeated sequences at each end of the chromatids
the repeats that make up a telomere are eaten away slowly over many division cycles, providing a buffer that protects the internal chromosome regions bearing the genes
How can chromosomes be analysed?
metaphase spreads requires actively dividing cells growth arrested in metaphase staining of chromosomes banding patterns dark patterns A T rich light patterns G C rich
ideograms
diagram of banding patterns of chromosomes
karyotyping
What are alleles?
alleles are different variants of genes
homologous chromosomes have the same genes but different variants (alleles)
sister chromatids have identical alleles
this is because the maternal and paternal copy of the gene can be different
How many chromosomes are there in mitosis?
chromosome number stays the same, you have one round of replication and one round of division
Describe what happens in prophase
chromosome condensation
nuclear envelope breakdown
centrosome separation
spindle fibres appear
Describe what happens in prometaphase
spindle fibres attach to chromosomes
chromosomes condense
centrosomes move to poles of cell
Describe what happens in metaphase
chromosomes align at the metaphase plate
Describe what happens in anaphase
centromeres divide
sister chromatids move to opposite poles
Describe what happens in telophase
nuclear envelope reforms
chromosome decondensation
spindle disassembly
Describe what happens in cytokinesis
cytoplasm divides
parent cell becomes two daughter cells with identical genetic information
What is nondisjunction?
missegregation of chromosomes
leads to aneuploidy (an unusual amount of chromosomes)
sister chromatids don’t separate and the whole of the chromosome goes to one end of the cell
What happens in first post-zygotic division nondisjunction?
leads to non mosaic phenotype
the cell with the extra chromosome will continue to go through mitosis
the cell missing the chromosome will die off
What happens in later post-zygotic nondisjunction?
leads to mosaicism, the presence of two or more cell lines in an individual
How many chromosomes are present in meiosis?
number of chromosomes reduced
one round of replication
two rounds of division
Describe what happens in meiosis 1
prophase 1
centrosomes separate
sister chromatids of a chromosome (eg maternal copy) and the sister chromatids of its homologue (eg paternal copy) have crossing over of genes to produce daughter chromosomes
metaphase 1
homologous pairs align on the metaphase plate and form a tetrad
random assortment of chromosomes
anaphase 1
homologous pairs separate to each pole
telophase 1
nuclear membrane appears
the daughter cells form as the cytoplasm divides and they are ready for another division
What are the consequences of meiosis?
generation of genetic diversity, from:
crossing over of genetic material in prophase 1
random assortment of chromosomes in metaphase 1
maintaining constant chromosome number from generation to generation
Describe what happens in meiosis 2
prophase 2
chromosomes condense
the nuclear envelope breaks down
centrosomes move apart
metaphase 2
the chromosomes line up individually along the metaphase plate
anaphase 2
the sister chromatids separate and are pulled towards opposite poles of the cell
telophase 2
nuclear membranes form around each set of chromosomes
chromosomes decondense
What is the difference in spermatogenesis and oogenesis?
from one spermatocyte to 4 sperm
from one oocyte to 1 egg and 3 polar bodies
What are the consequences of faulty meiosis?
nondisjunction leads to aneuploidy in gametes and monosomy and trisomy in the zygote 1/3 of identified miscarriages infertility leading cause of mental retardation
What happens in meiotic nondisjunction?
in meiosis 1
ends up with gamete having extra copy of that chromosome
after meiosis 2, form either 50% trisomy or 50% monosomy
in meiosis 2
ends up with gamete having one cell with extra copy and one missing it
forms 25% trisomy, 25% monosomy or 50% normal zygote
