Chapter 3 - Cell Division & Chromosome Heredity

Diploid/2N

Two copies of genetic material, two homologous chromosomes, same genes in same order

Haploid/N

One copy of genetic material, nonhomologous
E.g. human gametes

Diploid dominant life cycle

Humans - diploid phase is long

Haploid dominant life cycle

Most fungi - haploid phase is the mature form

Blend of diploid/haploid dominant life cycles

Some algae - sporophyte (2N) and gametophyte (N) stages

Mitosis is how ________ cells replicate

somatic

Meiosis produces ___________ cells

haploid gametic

M phase

Cell division occurs

Interphase

Period between cell divisions

G0 phase

Cell division stops, cell is specialized but no longer divides & eventually dies

S phase of interphase

Chromosome duplication & synthesis of DNA

Prophase

Mitotic spindle begins to form
Asters extend microtubules
Chromosomes are joined by a centromere

Prometaphase

Nuclear envelope disappears
Non-kinetochore microtubules begin to line up chromosomes along center of the cell

Metaphase

Chromosomes align along the metaphase plate
Spindles reach from centriole to chromosomes

Anaphase

Chromosomes are pulled in opposite directions

Telophase

Nuclear envelopes reform
Cleavage furrow appears

Cytokinesis

Splitting of cells

DNA content

G1 and mitosis phases are roughly equal in

S phase has double the...

chromatids

G1 check point

Adequate cell size, nutrient availability is sufficient, growth factors are present

S phase check point

DNA replication is complete, screened to remove base pair mismatch/error

G2 check point

Cell size is adequate and chromosome replication is successfully complete

Metaphase check point

All chromosomes are attached to mitotic spindle

Meiotic cell division begins with a...

diploid cell after chromosome duplication

Meiosis I results in...

two cells each with one isolated homologous chromosome

Meiosis II results in

four haploid gametes

Recombination

More common at the ends of chromosomes

Mutation

More common in males

Lowest allelic diversity

After meiosis II in the four haploid daughter cells

Oogenesis

Formation of egg cells

7 million

Number of primary oocytes in female fetuses

Prophase I

Primary oocytes stay in this stage until puberty

Metaphase II

During ovulation oocytes move to this stage

Meiosis II in oocytes

Only completed if fertilization occurs

Law of Independent Assortment & Meiosis

After many rounds of meiosis, there are ~equal proportion of each pair of alleles (GR, Gr, gR, gr)

500

Number of mature oocytes produced over lifespan

60-80k

Number of oocytes per ovary at the beginning of puberty

1-2 million

Number of primary oocytes at birth

Atresia

Form of apoptosis causing number of oocytes to decrease during fetal maturation

1 gamete & 2-3 polar bodies

Oogenesis results in

4 gametes

Spermatogenesis

Sperm

Motile gamete

Egg

Non-motile gamete

Oogenesis cell division is...

Unequal

Spermatogenesis cell division is...

equal

~70 days

Growth phase for spermatogenesis

Years-decades

Growth phase for oogenesis

Millions

Number of sperm produced each day

One

Number of eggs produced during each menstrual cycle

Homozygous

Alleles are the same

Heterozygous

Alleles are different

Hemizygous

Only one allele

Reciprocal cross

Allows one to examine effects of sex on phenotype, same cross with different sexes

Males express recessive phenotype because they have one X chromosome

X linked disorders in humans

Colour blindness, hemophilia A, Duchenne muscular dystrophy

Non-disjunction

Failure of sister chromatids to separate during meiosis, can happen at meiosis I, II, or both

Kleinfelter syndrome (XXY)

Male phenotype, tall stature, delayed or absent puberty, low muscle tone, small penis & testes
Could be caused by XX egg or XY sperm

Turner syndrome (XO)

Female phenotype, short stature, webbed neck, delayed or absent puberty, infertility
Could be caused by egg without X or sperm without X

Jacobs syndrome (XYY)

Male phenotype, variable symptoms, tall stature, curved pinky finger, widely spaced eyes, behavioral disorders, delayed development of social language & learning, large head, large testes, large feet, most are fertile

Trisomy X (XXX)

Variable symptoms, some developmental delays, taller than average, some kidney problems

XYYY syndrome

Very rare, similar symptoms to XYY

X tetrasomy (XXXX)

Tall stature & mild intellectual disability

YO syndrome

Not viable

Down syndrome (trisomy 21)

~0.3% of newborns & ~25% of spontaneous abortions
Most common mental disability
Least severe autosomal trisomy

Edwards syndrome (trisomy 18)

Severe intellectual disability, decreased muscle tone, low-set ears, internal organ defects

Patau syndrome (trisomy 13)

Severe intellectual disability & other problems
~90% die within first year of life

Autosomal trisomies other than 21, 18, & 13

Occur but are not viable

Crossing over

More frequent in males

Non-disjunction is sometimes associated with...

Chromosomes that did not recombine resulting in improper chromosomal segregation

Other mechanisms of non-disjunction

Checkpoint failure, age related degradation of cohesion complex

Chance of trisomy

Positively correlated with older mothers

Y linked inheritance

Male-male transmission, every son will have the gene/trait

~800

Number of genes on human X chromosome

~80

Number of genes on human Y chromosome

X inactivation

One X chromosome is randomly inactivated & varies throughout the cells of a multicellular organism
E.g. Calico cats, orange & brown express on different areas of female cat (or XXY male cat)

XIST

X linked gene required for X inactivation
Encodes long RNA molecule that covers chromosome to be inactivated

XIST transcript

acts in cis - only affects the chromosome from which it is transcribed

X upregulation

Males produce twice the X coded proteins in order to cause X dosage to be equal in males and females

Dosage of X does NOT have to be the same as...

autosomes