Lecture 21 Chromosomal basis of inheritance

diploid number for humans

2n
46

haploid number for humans

n
23

we cannot infer from the number of chromosomes (haploid number) of a species:

# of genes
organism size
complexity

chickens have __ as many chromosomes as humans

almost twice

chromosome number and arrangement can change over evolutionary time

human chromosome 2 is result of fusion bw two ancestral ape chromosomes

in euk, transmission of genetic material from one gen to another involves

mitosis and meiosis

mitosis leads to production of

2 daughter cells
each with same # chromosomes as parent cell
2n-->2n (in diploids)

meiosis leads to production of

gametes
each with half number of chromosomes
2n-->n

interphase is a ____ phase

non-division phase

cell cycle

alteration between division (mitosis) and non-division (interphase)

G0 stage "G zero

the quiescent stage
a quiescent cell is neither dividing nor preparing to divide, but may later re-enter the cell cycle

S phase

part on interphase
cell's DNA is replicated to produce two identical copies

during most of the cell cycle, DNA exists in an unfolded state known as

chromatin

in preparation for cell division, DNA is replicated and condensed into

chromosomes

1 chromosome can be defined as:

1 single DNA molecule (condensed)
2 sister chromatids joined by centromere

since mitosis occurs after DNA replication, each mitotic chromosomes contains

two identical DNA molecules
these are "sister chromatids

centromere

connects sister chromatids
where spindle fibers will attach to the chromosome during mitosis

centromere location

can vary between different chromosomes

structure of condensed mitotic chromosome

sister chromatids
centromere
p arm (top)
q arm (bottom)

chromosomes exist as ______ in diploid organism

homologous pairs
one homolog is "maternal" the other is "paternal

human karyotype

contains two of each chromosome (except X and Y)

homologous chromosomes carry the same ___ but may carry different ___

same genes
diff alleles

DNA replication produces

identical sister chromatids (for each chromosome)

a human cell just prior to mitosis contains __ chromosomes and __ chromatids

46 chromosomes
92 chromatids

interphase

not part of mitosis
chromosomes are extended and uncoiled, forming chromatin
cell spends most of its time here
DNA replication occurs during interphase (S phase)

prophase

chromosomes condense, sister chromatids are already attached at the centromere
nuclear envelope breaks down
centrioles migrate to opposite poles

(prometaphase and) metaphase

spindle fibers form
chromosomes align at metaphase plate

anaphase

centromeres split, sister chromatids separate (disjunction)
sister chromatids are now daughter chromosomes
daughter chromosomes migrate to opposite poles

telophase

daughter chromosomes arrive at opposite poles
cytokinesis (division of cytoplasm) occurs
chromosomes decondense and nuclear envelope re-forms