Ch 1: Chromosomes

In eukaryotic cells, nuclear DNA is present as ____ chromosomes inside of the nucleus

linear

What is the distinguishing feature of eukaryotic cells?

the nuclear envelope

In eukaryotic cells, organelle DNA is present as ____ chromosomes inside of the _____ and, in plants and algae,_______

circular, mitochondria, chloroplast

What are the two major phases of cell division?

- M phase (mitosis + cytokinesis)

- interphase

Mitosis

division of the nucleus

cytokinesis

division of the cytoplasm

interphase

- G1 (cell growth) +S (DNA replication) +G2 (cell growth)

- non-divisional phases of cell division cycle

G0

- cell may exit cell division cycle during G1 and go into G0 to become non-dividing cell (quiescence)

- intestinal epithelium, neurons, etc (stay in G0 until death)

-if part of liver is cut, G0 cells will enter diving phase again to replace tissues

G1/S checkpoint

- cell makes sure dna molecule is intact, if not it fixes it and moves on

- once cell undergoes s phase the cell must divide

G2/M checkpoint

- makes sure all molecules replicated and size is large enough

end of mitosis and during G1/G0

each chromosome consists of a single dna molecule

after S-phase and during G2

each chromosome consist of two DNA molecules (two identical sister chromatids)

telomeres

- stable ends of linear chromosomes

- bacteria are circular and do not have telomeres

centromere

- defined region of a chromosome at which sister chromatids are joined following dna replication

interphase vs prophase

interphase:

- chromatin is decondensed and unidentifiable

- bowl of noodles (own distinct territories- don't like to mix, gaps for enzymes to travel)

prophase:

- chromatin is condensed into visible chromosomes by tightening interactions between dna and chromatin proteins (histones- wraps around inside particle twice and held in place by outside histone)

nucleosome

- basic unit of organization of dna in nucleus

- dna wrapped around 2x + 2 histones

chromatin

- dna+histones but loose/decondensed

- occurs during non-divisional phases of cell cycle

chromosomes

- before rep: single molecule of dna

- after rep: chromosomes consist of 2 sister chromatids, which contain identical dna sequences

chromosomes classified based on centromere location

- metacentric: center

- submetacentric: near center

- acrocentric: near telomere

- telocentric: at the telomere

- point of junction rarely at the center of the chromosome

locus

- specific point in chromosome

- p arm: petite/short arm

- q arm: long arm, below centromere usually

- Chromosome #, arm, designation

- all genes are in loci, but not all loci contain genes

haploid

-N

- one copy of genetic material subdivided into chromosomes

- unpaired chromosomes

- humans= 23, gametes only

Diploid

- 2N

- two copies of genetic material subdivided into chromosomes

- diploid organism has 2 sets of chromosomes organized as homologous pairs

alleles

alternative forms of gene found at same position (locus) of homologous chromosome

homologous chromosomes

- chromosomes with same length and centromere location but not necessarily identical dna sequences. some of their genes may be allelic

- not sister chromatids because not the product of replication of one chromosome, not joined, and not identical

gene (transmission genetics)

a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.

gene (molecular genetics)

a region of a chromosome that codes for a functional product

karyotype

- Visual representation of an organism's complete set of chromosomes.

- humans- 22 homologous autosomal and pair of sex determining chromosome

Ploidy

the number of sets of chromosomes in an organism

polyploid

3n,4n,5n, etc

aneuploid

- Abnormal number of chromosomes.

- humans, having any number of chromosomes that is not 46

kinetochore

- protein structure that is assembled on centromere

- during mitosis, spindle microtubules attach to it and mediate the disjunction of sister chromatids

centrosome

- pair of centrioles

- it is the microtubule-organizing center of animal cells

- it is replicated before the beginning of mitosis, becoming a pole for spindles

prophase

- chromatin condenses into visible chromosomes

- centrosomes move away from each other to become poles of mitotic spindles made from microtubules

prometphase/metaphase

- nuclear envelope breaks down, mitotic spindles attach to kinetochores at the centromeres of chromosomes, and chromosomes line up at metaphase plate

anaphase

- chromatids are pulled apart by spindles and are disjoined

- are now individual chromosomes

- transient 4n cell (4 chromosomes becomes 8/ humans 23->92)

telophase

- chromosomes are segregated and their dna decondenses

- they become the chromosomes of the daughter cells

- nuclear envelope reassembles

- 2 diploid (2n) cells, each chromosome consist of single dna molecule

defining feature of mitosis

- all chromosomes go to metaphase plate individually

- homologous chromosomes at metaphase plate is not a requirement (haploids can undergo mitosis)

meiosis

- a special type of eukaryotic cell division cycle that occurs during generation of gametes in animals (sperm/eggs), or spores in plants and single-cell eukaryotes

- occurs in germ cells

- 2 rounds of division-> 4 haploid (1n cells)

tetrads

- 2 homologous chromosomes together (23 tetrads in humans)

meiosis 1

- reduction division

- tetrads are disjoined

- separated homologous chromosomes, still with 2 chromatids each, are segregated to daughter cells

- number of chromosomes in two daughter cells is half of that of the original cell

crossing over

-exchange of fragments of DNA between paired homologous chromosomes, occurs during pachytene

- pairing of chromosomes mediated by synaptonemal complex (ordered protein structure formed between homologous chromosomes during meiotic prophase 1)

- complex facilitates crossing over between non-sister chromatids and disappears during late meiotic prophase 1

5 substages of meiotic prophase 1

- leptotene: DNA begins to partially condense

- zygotene: homologous chromosomes pair to form tetrads (bivalents)

- pachytene: condensation continues and the individual chromatids become more visible. Crossing over occurs: the exchange of fragments of DNA between the paired homologous chromosomes

- diplotene: the homologous chromosomes begin to partially separate. The chiasmata become visible: points at which crossing over already occurred

- diakinesis: the homologous chromosomes pull further apart (but they remain together). Further condensation occurs in preparation for metaphase

following pachytene

- sister chromatids are no longer identical dna molecules due to crossing over and recombination between non-sister chromatids

middle prophase 1

chromosomes begin to condense and the spindle forms

late prophase 1

Homologous chromosomes pair, crossing over takes place, nuclear membrane breaks down

metaphase 1

Paired homologous chromosomes line up along the metaphase plate, mitotic spindles attach at kinetochores

anaphase 1

homologous chromosomes separate and are pulled to opposite ends of the cell

telophase 1

- 2 haploid (1n) cells

- chromosomes arrive at spindle poles and cytoplasm divides

meiosis 2

- equational division

- chromatids are disjoined and become individual chromosomes

- they are segregated to daughter cells

prophase 2

- may or may not decondense

- chromosomes recondense

metaphase 2

-individual chromosomes line up on the equatorial plate

anaphase 2

sister chromatids separate and move toward opposite poles

telophase 2

- chromosomes arrive at spindle pole and cytoplasm divides -> outcome is 4 haploid cells

- need diploid or even number for meiosis to be able to occur

spermatogenesis

- equal cytokinesis

- 4 gametes (sperm=haploid)

oogenesis

- unequal cytokinesis- one cell receives all the cytoplasms

- rest become polar bodies

- 1 gamete (egg=haploid)

fertilization

produces a zygote (diploid)

nondisjunction

- occurs if chromosomes or chromatids do not separate during anaphase 1 or anaphase 2 of meiosis

- both chromosomes/chromatids migrate to same gamete

- after fusion with normal gamete, trisomic and monosomic aneuploidies may occur

down syndrome

- trisomy 21 aneuploidy

- trisome-x viable but 13 and 18 not usually

turner syndrome

- monosomy x aneuploidy

- female features with webbed neck, shield chest, underdeveloped breast, rudimentary ovaries, brown nevi

cri-du-chat syndrome

- loss of piece of one of two copies of a chromosome, partial monosomy of 5p

- complete autosomal monosomies not viable in humans