Why are cells small?
Surface to volume ratio
Diffusion of wastes and nutrients.
Chromosomes
The DNA molecule exists for most of the cell cycle as chromatin, a long, uncoiled single strand.
Chromatin will coil around proteins called histones to form chromosomes.
Chromosomes are packages of DNA which can be transferred to new daughter cells when the cell reproduces through mitosis.
Chromosome Structure
Homologous chromosomes - contrasting chromosomes of same type, one from each parent.
Chromatid - one half of the "X" chromosome shape. The two halves are sister chromatids and exact copies of each other. They will go to separate daughter cells during meiosis.
Centromere - protein band that joins the two sister chromatids.
Human body cells contain 46 chromosomes
23 pairs
22 pairs of autosomes
1 pair of sex chromosomes
X chromosome from mom’s egg
X or Y chromosome from dad’s sperm
= XY, = XX
Sex-linked traits have a loci (singular locus) on the X chromosome, so that only one recessive allele is required for expression in males, since males have only one X chromosome.
Somatic (body) cells are
diploid (2n) - two sets of homologous chromosomes, one from mom, one from dad.
Sex cells (gametes) are
haploid (n) - one set, no homologies
Sex cells are formed through the process of meiosis.
Multiple allele traits are those traits which are governed by more than one single allele (or gene)
A karyotype is a print of the chromosomes of a cell. It is a diagnostic tool used to evaluate the total # of chromosomes in the cell.
Nondisjunction - the failure of two homologies to separate during meiosis, resulting in a condition known as trisomy (three bodies) =
2n +1 = 47 chromosomes in humans.
Down’s syndrome (mongolism) is trisomy 21, of the 21st chromosome pair.
Aneuploidy - the condition of missing a chromosome due to nondisjunction:
2n - 1 = 45 chromosomes.
Turner's syndrome X__