genetics ch. 3

describe G1 & G1 checkpoint

active gene expression & activity, prep for synthesis, longest phase of cell cycle

check for adequate cell size & nutrients, growth factors present

describe S & S checkpoint

DNA replication, chromosome duplication

check that DNA replication is complete and accurate

describe G2 & G2 checkpoint

prepare for cell division, organelles and proteins necessary for cell division produced

check for adequate cell size, chromosome replication completed

what happens in the M checkpoint

check that all chromosomes are attached to the mitotic spindle

what are the steps of mitosis

1. prophase

2. prometaphase

3. metaphase

4. anaphase

5. telophase

what happens in prophase (mitosis)

early mitotic spindle & aster form, chromosomes begin to condense, centromeres appear

what happens in prometaphase (mitosis)

nuclear membrane gone, kinetochore/nonkinetochore microtubules connect, centrosomes are at poles

what happens in metaphase (mitosis)

max. chromosome condensation, chromosomes line up at the metaphase plate, spindle apparatus connected & ready

what happens in anaphase (mitosis)

separase & disconjunction separate sister chromatids, move towards the poles by depolymerization

what happens in telophase (mitosis)

nuclear membrane reassemble, chromosomes decondense, two 2n nuclei in an elongated cell, cytokinesis next

what is the centromere

sequence on the chromosome where sister chromatids are joined

• where the kinetochores bind

what is the centrosome

organelle containing/creating the microtubules —> nucleate the spindle fiber microtubules

(+) ends are far ends, grow toward kinetochores/chromosomes

(-) ends are located at the centrosome

spindle fibers emanate from centrosome in a pattern called the aster

what are the three types of microtubules

kinetochore microtubules, polar/nonkinetochore microtubules, astral microtubules

describe kinetochore microtubules

attach centrosome to kinetochores on chromosomes

depolymerize at the (+) end during separation of chromosomes

attach (+) end to kinetochores

kinetochores contain fibers with motor proteins

describe polar (non-kinetochore) microtubules

attach centrosome to centrosome

function in cell elongation & stability

also divide

motor proteins connect ends (+ to +)

describe astral microtubules

attach centrosome to membrane

function in cell stability and shape

what is cohesin

4-subunit protein complex that acts as glue between sister chromatids

most is located at centromeres but they go along the whole chromatid

prevent premature separation

sis. chromatid. cohesion (cohesin) balance tension by kinetochore microtubules pulling, creates alignment at the metaphase plate

what is separase

cleaves Scc1 - component of cohesin complex

causes disjunction

describe cytokinesis (in animals)

actin contractile ring, cleavage furrow

begins on the edges and moves inward

describe cytokinesis (in plants)

new cell wall created along the cellular midline, called the cell plate

begin in the middle and move outward

how are checkpoints monitored

monitored by protein interactions, determine readinesss for next stage

cyclin-dependent kinases & cyclins

what are the main events/characteristics of meiosis 1

2n —> n “reductional division”

homologs separate

genetic diversity: crossing over between homologs, segregation of homologs

centromeres do NOT split

what are the events of prophase 1 (meiosis)

1. leptotene

2. zygotene

3. pachytene

4. diplotene

5. diakinesis

lazy zebras prefer dancing daily

leptotene

chromosomes condense, meotic spindle forms (microtubules extend from centrosomes)

zygotene

condense, nuclear membrane breakdown, homologs enter synapsis

pachytene

condense, tetrads visible, crossing over occurs, recombinant nodules form, microtubules attach

diplotene

condense, synaptic complex dissolves, chiasmata revealed, cohesin is present

diakinesis

kinetochore microtubules align homologs at metaphase plate

what happens in metaphase 1 (meiosis)

homologs align at opposite sides of metaphase plate, microtubules attach to kinetochores

what happens in anaphase 1 (meiosis)

homologs are separated by depolymerization of kinetochore microtubules

cohesin between sister chromatids is intact

what happens in telophase 1 (meiosis)

nuclear membranes briefly reform around haploid sets of chromosomes

what does cytokinesis 1 (meiosis) yield

2 haploid cells, chromosomes = sister chromatids

synaptonemal complex

tri-layer protein structure forms and binds non sister chromatids of homologs, facilitates crossing over

recombinant nodules

form at intervals within the synaptonemal complex, they are aggregates of enzymes/proteins needed for crossing over

chiasma

chiasmata (plural)

location where crossing over has occurred

what are the main events/characteristics of meiosis 2

n + n —> n

sister chromatids separate

4 haploid gametes produced

what does cytokinesis 2 yield

4 haploid gametes, chromosome = single chromatids

what are the events of meiosis 2

very similar to mitosis steps, but result in genetically diverse gametes rather than daughter cells

prophase 2, metaphase 2, anaphase 2, telophase 2

what is the main reason for genetic diversity

INDEPENDENT ASSORTMENT - alleles segregage with equal probability of each arrangement of paired homologs

difference between chromosomal and phenotypic sex

chromosomal - chromosomes unique to each sex, determined at fertilization

phenotypic - morphology of each sex, difference in gene expression

both of these determine sex overall

XY system

sex determination in mammals

males XY (XXY, XYY) females XX (XO, XXX)

contain pseudoautosomal regions

embryos have undifferentiated gonadal tissue - determined by presence of SRY gene during development

what are pseudoautosomal regions

areas of homology between X and Y chromosomes, allow homologous pairing during meiosis

PAR 1 & PAR 2

crossing over occurs here

describe SRY gene function

SRY is a transcription factor needed for male-specific gene expression

located on the Y chromosome

SRY expressed - cause testicular development, male (Wolffian duct)

SRY not expressed - default ovarian/uterus development, female (Müllerian duct)

ZW system

males ZZ, females ZW

birds, butterflies, moths, some reptiles and fish

dosage system

X/Autosome ratio

more X = female, X/A = 1, two X

less X = male, X/A = 0.5, one X

drosophila

describe dosage compensation and give examples

change in expression that compensates for difference in number of copies of genes between males and females

• double expression of genes in sex where they are lacking one (XY double of X) - fruit fly male

• each of decreased to ½ expression - roundworm female

• paternal X-inactivation - marsupial mammals

• random X-inactivation - placental mammals

  • also known as “Lyon hypothesis”

  • inactive X becomes Barr body at edge of nucleus

  • x inactivation is permanent for all descendants of first cell

  • “mosaics” - some paternal X inactivated, some maternal X inactivated in the cells of a female mammal

visible mocaisism examples

calico & tortoiseshell cats

x-chromosme has gene for coat color

random x-inactivation in heterozygous females causes unique patterns of different colors. one allele for black one allele for yellow

mechanism of X-inactivation

Xist gene = X-inactivation-specific-transcript

produce large RNA molecules that cover inactive X

can only act on the chromosome from which it is transcribed (cis)

some genes escape inactivation

what is hemizygous

only one copy of a gene is present. typical in males (XY, only one copy of X and Y), also observed in gene deletions

hemizygous males display any allele present on single X chromosome, whether it is recessive or dominant

this causes X-linked recessive traits to be much more prevalent in males

in X-linked dominant traits, heterozygous females and males hemizygous for the dominant allele express dominant phenotype

describe recessive X-linked

expressed more often in MALES - due to hemizygosity

X inherited from mother

females often carriers that do not express trait

examples: colorblindness, hemophilia, muscular dystrophy

Hemophilia A

x linked recessive

mutation on factor VIII gene on X chromosome (clotting factor)

causes nonfunctional protein

de novo queen Victoria

describe dominant X-linked

EQUAL expression in males & females

affected dads pass on to all daughters

examples: congenital hypertrichosis (many hair follicles & body hair)

describe Y-linked

transmit exclusively from males to males