Download chromosome

mitosis = nuclear division that produces two daughter cells with thesame number and kinds
of chromosomes as the parental cell (cell that divides)
chromosome = condensed DNA in the form of a chromatid
-in the dividing cell - chromosome duplicates and is found in the form of two sister
chromatids joined by a centromere
cell cycle = division of the cell, consists of interphase, mitosis and cytokinesis
cytokinesis = division of the cytoplasm and organelles of a cell that has undergone mitosis
into two daughter cells
chromatin = interphase form of DNA, DNA helices associated with proteins = histones
-each cell (except sex cells) has two complete sets of 23 chromosomes
-therefore the cell is said to be diploid and n = 23 (2n = 46)
Interphase: DNA begins to condense, duplicate and
form the chromosomal structure (2 chromatids
+ centromere)
-each cell = 2n (where 2 = 23)
Prophase:movement of centrioles (specialized
groups of microtubules) to the opposite ends
(poles) of the cell
-appearance of spindle fibers between the
centrioles
-fragmentation of the nuclear envelope,
disappearance of the nucleolus
-complete formation of the spindle = asters (short
microtubules from the centrioles), poles with
centrioles, and spindle fibers
Metaphase: alignment of chromsomes along the
spindle
Anaphase: separation of sister chromatids (called
daughter chromosomes)
-movement of chromosomes to the opposite
poles- by the spindle (lengthening and shortening
of spindle fibers)
Telophase: decondensing of DNA, cytokinesis
by the formation of a cleavage furrow
Cytokinesis: by the formation of a cleavage furrow
-furrow - slight indentation around the circumference of the cell
-actin filaments then assemble to form a contractile ring which decreases in size
“purse string” mechanism
-pinches the cell in half
-genetic diversity at mitosis
provided by crossing over
-between non-sister chromatids of 4
chromosomes (tetrad)
-results in an exchange of genetic
material and diversity
4n = 8
2n = 4
-like mitosis but requires 2 nuclear divisions - cell undergoes meiosis I and II
-occurs in the formation of sex cells and results in 4 daughter cells each with half the number
of chromosomes as the parental cell (haploid)
-ensures genetic diversity - crossing over, different combinations of chromosomes within
the resulting sex cells, fertilization results in different combinations
Meiosis I: similar to mitosis EXCEPT at anaphase
where duplicated chromosomes DO NOT separate
-they travel to the poles as a pair of chromatids
-Prophase I - duplication of chromosomes and
attraction of homologous pairs of chromosomes
(synapsis) forming tetrads
-occurrence of crossing-over
-Metaphase II - alignment
-Anaphase II - separation of chromosome pairs
Meiosis II: similar to mitosis
-lining up of chromosomes at Metaphase II
-separation of sister chromatids at Anaphase II
-cytokinesis at Telophase II
-production of sperm and egg occurs
through meoisis
-unequal division of the primary oocyte (2n)
forms the secondary oocyte and a polar body
-polar body - contains chromosomes but
very little cytoplasm
-polar body splits during oogenesis
-secondary oocyte develops into the ovum
(haploid)
-fertilization unites haploid sperm with the
haploid egg - produces diploid individuals
-polyploidy = extra pair of chromosomes
-one gamete is diploid - non-disjunction
during meiosis I (homologous pairs fail
to separate) or meiosis II (sister chromatids)
-results in a triploid cell
e.g. trisomy 21 = Down’s syndrome (1/770)
trisomy 13 = Patau’s syndrome (1/15000)
trisomy 18 = Edward’s syndrome (1/4000)
trisomy 22 = very rare
-aneuploidy = missing a chromosome pair
-occurs in sex chromosomes more
frequently
-extra chromosome 21
-from birth - each individual is diploid (containing two copies of the 23 different chromosomes
-chromosome charts = karyotypes (display of the 23 chromosome pairs)
-pairs 1 through 22 are called autosomes = do NOT determine sex
-pair 23 = sex chromosomes (X and/or Y)
-diseases: Huntington’s - chromosome #4
Cystic fibrosis - chromosome #7
Sickle cell anemia - chromosome #11
Tay-Sachs disease - chromosome #15
Amyotrophic lateral sclerosis - chromosome #21
-because we have two copies of each chromosome - we have two copies of each gene
-multiple forms of each gene = alleles
-so homologous chromosomes are the same gene for gene - BUT they can be made up of
different alleles of that gene
-dominant allele - uppercase letter e.g. E
-recessive allele - lowercase letter e.g. e
-individual with two identical alleles of one gene on each chromosome = homozygous
- individual with two different alleles of one gene on each chromosome = heterozygous
e.g. allele E or e
-union of sperm and egg with the same alleles = homozygous = EE or ee
-union of sperm and egg with different alleles = heterozygous = Ee
-genotype: genetic makeup of an individual
-phenotype: physical or biochemical manifestation of the genotype
-dominant trait: results from the combination of a dominant allele with
another dominant allele (homozygous dominant) or with a recessive
allele (heterozygous dominant)
-recessive trait: results from the combination of two recessive alleles
(homozygous recessive)
-Punnett square: frequently used to determine the phenotype of offspring
and the odds that a genotype will occur
-75% chance of a dominant
trait resulting
-25% chance of a recessive
trait resulting
-results when one allele is not
completely dominant over another
-results in an intermediate phenotype
-can also be known as ‘blending’
e.g. sickle cell anemia
-irregular RBC shape
-caused by abnormal hemoglobin
(less soluble)
HbAHbA normal
HbSHbS sickle cell
HbAHbS carrier
-Autosomal recessive disorders: homozygous recessive
Cystic Fibrosis - 1/20 Caucasians (carrier - heterozygous), 1/2500 newborns
-mutation in gene that encodes a chloride channel protein
Tay-Sachs disease - Jewish descent
-lack of the enzyme hexosaminidase (HexA)
Phenylketonuria - 1/5000 newborns
-lack enzyme needed to process the amino acid, phenylalanine
-accumulation - neurological failure, mental retardation
-Autosomal dominant disorders: homozygous or heterozygous dominant
Neurofibromatosis - 1/3000 people, equally represented
-café-au-lait spots on the skin, mostly mld symptoms
-severe symptoms - skeletal deformaties, large head, eye and ear tumors
-gene on chromosome 17, controls cell division
Huntington disease - 1/20000 people
-progressive degeneration of neurons
-gene on chromosome 4 - no known cure
-Sex-linked disorders: contain alleles of genes also
-X-linked - contributed by mother
e.g. color blindness, muscular dystrophy, hemophilia
-Polygenic disorders: controlled by more than one allele
e.g. cleft lip, clubfoot, hypertension, diabetes
-more than one allele can control normal traits
e.g. skin color
AABB
Very dark skin
AABB or AABb Dark skin
AaBb, AAbb or aaBB Medium skin
Aabb or aaBb Light skin
aabb
Very light skin