Lecture 6: Chromosomes and Mitosis

what is hereditary info found on

genes

where are genes located

on chromosomes

t/f: eggs and sperm contribute equally to the genetics of offspring through their nuclei

true

what are karyotypes

pictures of every chromosome in a cell

what can be seen about chromosomes in karyotypes

how many chromosomes the organism has (including mono/di poloidy, etc), size, number, shape, and banding pattern

how would you expect chromosomes of a diploid organism to appear in a karyotype

in pairs

in what cell phase and in which cells are pictures to be used to make karyotypes taken in

metaphase chromosomes and in somatic cells (NOT GERM CELLS) → won’t have all the chromosomes (half)

what are autosomes

all chromosomes except x and y (sex chromosomes)

what are sex chromosomes

x and y chromosomes (1 pair in humans)

humans with which chromosomes are assigned male vs female at birth

male - xy

female - xx

t/f: having (only) 2 x chromosomes always means you are born female (across species)

no → sex determination isnt the same in all species

are males or females heterogametic vs homogametic, etc

males - heterogametic

females - homogametic ( 2 of same sex chromosome)

t/f: females are always the homogametic sex

false

explain the difference in human vs drosophila (fruit flies) in sex determination

both: typical male = xy, typical female = xx

humans: as long as there is >1 Y, the individual is male

• Y chrom determines sex

drosophila: at least 2 X chroms, the individual is female

• X chrom determines sex

t/f: individuals can have more or less than two sex chromosomes

true

what is a good rule of thumb to determine, in humans, if individuals will be male if they have more than one chromosome

as long as there is at least one y, the individual is male

t/f: humans need at least 1 sex chromosome to be viable

true

what are the 2 major processes of cell division

mitosis and meiosise

what is mitosis

nuclear division that results in 2 daughter cells → each containing identical numbers of chromosomes to parent cell

• produces clones of parent cell

what is meiosis

nuclear division that results in 2 daughter cells, each containing half of the number of chromosomes found in other (somatic) cells

produces cells that are unique compared to parent cell

what are diploid cells

• have 2 complete sets of chromosomes

• most body cells are diploid

• mitosis produces diploid cells from a diploid parent cell

what does meiosis do

produces haploid (n) gametes (sperm or eggs)

• haploid cells (daughter cells in this case) have one copy of each chromosome

what does n stand for in genetics

n = number of chromosomes in a normal gamete

what is the diploid and haploid number in humans

diploid: 2n = 46

haploid: n = 23

what is the diploid and haploid number in drosophila

diploid: 2n = 8

n = 4

what is a chromatid

one copy of a newly copied chromosome that is still joined to the other copy by a single centromere

what are nonhomologous chromsomes

• contain different genes

• do not pair at meiosis

what are homologous chromosomes

• members of a chromosome pair

• contain the same genes (can be different alleles, though)

• pair at meiosis

sister chromatids

• identical copy of an individual chromosomes

• identical alleles for all genes

how long would the DNA molecule that makes up each human chromosome be

what is the cell cycle

an alternation between interphase (normal cellular activity) and mitosis (cell division)

what is the G0 stage

when some terminally differentiated cells (eg mature neurons) stop dividing and enter G0 stage

how long does the cell cycle take

• rapidly proliferating human cells go through the cell cycle in ~24 hours

• in yeast, the cell cycle is ~90 minutes

• in an early embryo, its ~30 minutes

explain the G1 phase

Part of interphase (interphase gap/growth phase 1)

• cells are actively producing products specific to their biological role in the body

• varies in length, depending on cell type

• some cells (eg. human nerve cells) do not divide and stay in G1 (in which case it gets termed G0)

• The cell is metabolically active, growing, and preparing for DNA replication.

• In G₁, the cell:

  • Increases in size

  • Synthesizes proteins and organelles

  • Checks for DNA damage before entering S phase (where DNA is duplicated)

explain S phase:

part of interphase - DNA synthesis

• duplication (synthesis) of chromosomes to produce identical sister chromatids

• sister chromatids remain joined at the cemtromere

• DNA must be “open” for replication, not condensed (unwinds to create chromatin, wouldnt look like the standard pic of a condensed chromosome)

• in animal cells, centrosomes (structures involved in cell division) also need to replicate to produce two centrosomes

explain the G2 phase

part of interphase - Gap 2 Phase / Growth Phase 2

• phase just before mitosis

• cell synthesis proteins necessary for mitosis and cell division are produced

what are some key aspects of mitosis

• it is cell division resulting in 2 daughter cells

  • daughter cells are genetically identical to each other and parent cell

  • daughter cells have same number and kind of chromosomes as the parent nucleus

• Mitosis is typical of ordinary tissue growth or of asexual reproduction in single-celled eukaryotes

Explain the diff between Mitosis and Binary fission

Mitosis is a eukaryotic process

→ eukaryotic nuclei typically have multiple linear chromosomes

Prokaryote cell division / reproduction occurs via binary fission

→ procaryotes do not have nuclei and usually have a singular circular chromosome

what are the 5 phases of mitosis? briefly explain each

1. Prophase

• chroms condense and become visible

2. Prometaphase

• nuclear envelope (membrane surrounding nucleus) breaks down, spindle forms, and sister chromatids attach to microtubules from opposite centrosomes

3. Metaphase

• chroms align at cell equator

4. Anaphase

• sister chromatids separate and move to opposite poles

5. Telophase

• spindle disapears, chroms decondense, and are enclosed in 2 nuclei

explain prophase

• chroms condense and become visible

• 2 centrosomes (formed during S phase) move apart → each migrating to their own opposing pole of the cell

• microtubules (proteins) begin to extend from centrosomes

• nucleoli (regions of nucleus where ribosomes live) begin to disappear

what are centrosomes and what do they consist of

centrosomes are structures that consist of 2 centrioles plus other proteins (microtubules they expel)

explain the diff btwn centrosomes and centromeres

centrosomes consist of a pair of centrioles (structures that secrete microtubule proteins)

a centromere is the middle region of chromosomes where sister chromatids join

what are kinetochores

protein structures at. the centromere on chromosomes → the thing on centromeres that connect to the microtubules in prometaphase

explain prometaphase

• nuclear mem breaks down

• microtubules from centrosomes attach to kinetochores in the centromere of each sister chromatid

  • sister chromatids attach to microtubules from opposite poles

where do the 3 types of microtubules (kinetochore, polar, and astral) microtubules originate from respectively and what do they all work tg to form

all originate from the centrosome and form the mitotic spindle

what does mitotic spindle consist of

the 3 types of microtubules (kinetochore, polar, and astral)

what do kinetochore microtubules do

attach to kinetochores

what do polar microtubules do

are directed towards the middle of the cell from each pole, pushing them further away from each other in prometaphase

what do astral microtubules do

extend towards the cell’s periphery, help ensure spindles are oriented so they will pull chromosomes towards poles of cell

explain metaphase

chroms align on metaphase plate (middle of cell)

• sister chromatids face opposite poles of the cell

• forces pushing/pulling chroms to or from each pole are balanced, which keeps chroms in place

explain anaphase

sister chromatids of all chroms simultaneously separate at their centromere

• separated sister chromatids move to opposite poles via shortening kinetochore microtubules → disjunction

  • chromatids have characteristic “v shape” at this stage

explain telophase

nuclear mem (envelope) forms around each group of chromatids

• nucleoli re-form

• spindle fiber disappears

• chroms uncoil and reform as chromatin

explain cytokenisis

cytoplasm of parent cell splits into 2 daughter cells w identical nuclei

• beings during anaphase but isnt complete until after telophase

t/f: cytokenisis is diff in plant and animal cells

true → bc plant cell walls are too thick so cant “pinch off” (so they have a diff process)

summary of mitosis

t/f: the cell cycle is unregulated

false