BIOL100 Chapter 13

What do offspring inherit from their parents?

Genes; not traits

How do genes influence traits

genes are biological instructions

How do hydra asexually reproduce?

Process of budding: mass of mitotically dividing cells that eventually detach to become a clone organism

Asexual reproduction occurs in what kind of organisms?

unicellular and multicellular

limitation of asexual reproduction

low genetic variation; poor ability to adapt to changing environment → limits natural selection and evolution

product of fertilization of an egg and sperm cell

zygote of 46 chromosomes → multicellular organism

How are traits physically expressed through the genes in our chromosomes?

genes code for protein production

Genes are units of ___ and made of __

heredity, DNA

gene’s specific location on a chromosome is called…

locus

Sexual life cycle involves the alternating of these two processes

Meiosis: produces haploid gametes

Fertilization: produces diploid zygote

length of haploid vs diploid phases vary by…

organism; humans spend most other their lives in the diploid stage

What tool is used to visualize chromosomes in human somatic cells?

microscopy

vitro

process of cell growth and manipulation used in microscopy; uses tissue culture technique such that cell attaches to substrate and stops dividing when a density is reached

when are chromosomes most visible under microscopy? what is produced to visualize them and how is it used?

metaphase of mitosis (condensation of chromosomes)

metaphase spread; applied to cytogenetics studies

Karyotype

• shows 23 pairs of chromosomes of diploid human somatic cell

• first 22 pairs are autosomes, 23rd pair is sex chromosomes

  • XY = male and XX = female

• detects chromosomal abnormalities due to cell cycle errors (includes cancer)

in comparison to one another, homologous chromosomes of a pair are the same in…

length, shape, centromere position, gene loci

Why are X and Y chromosomes not fully homologous

differ in length and gene content; shared homology in some but not all regions

Why is homology essential for meiosis?

pairing and info exchange of homologous chromosomes

do sister chromatids change total chromosome count?

no

ploidy transition that cell undergoes in fertilization

haploid gametes (egg and sperm; n + n) → diploid zygote (2n)w

what does n equal?

pairs of chromosomes; ex n = 23 pairs, 2n = 46 pairs.

ploidy transition that cell undergoes in mitosis

diploid germ cell (2n) → 4 haploid gametes (each n)

what cells undergo meiosis to form gametes?

germ-line cels in ovaries and testes

why is shift between meiosis and fertilization important for sexually reproducing organisms?

maintains chromosome number across generations

Which two multicellular stages of the life cycle do plants alternate btwn?

sporophyte (2n): diploid stage

gametophyte (n): haploid stage

How are haploid gametophytes formed? (plants)

sporophyte (2n) undergoes meiosis → haploid spores (n) produced

haploid spores undergo mitosis → haploid gametophyte (n)

haploid gametophyte (multicellular organism; n) undegoes mitosis → haploid gametes

two rounds of mitosis of gametophyte (n) haploid phase

(1) spore → gametophyte

(2) gametophyte → gametes

plant fertilization produces ___ by…

diploid zygote (2n) thru fusion of two haploid gametes

How does zygote grow into a new sporophyte?

thru mitosis

which stage dominates plant life cycle?

sporophyte

Meiosis is also known as ___ division that converts a diploid cell into ___ haploid cells

reductional, 4

DNA replication results in

1 → 2 sister chromatids per chromosome

same number of chromosomes

same ploidy

around of DNA is doubled

Meiosis 1 is where reduction in ___ occurs

ploidy

Meiosis 2 is where reduction in ___ occurs, but ___ remains the same as produced in meiosis 1

chromatin content, ploidy

spermatogenesis

gametogenesis in males

• germ-lining cell of testes undergoes mitosis → spermatocytes (diploid precursor)

• primary spermatocyte (2n) undergo meiosis 1 → 2 secondary spermatocytes (n) w/ duplicated chromosomes

• secondary spermatocytes undergo meiosis 2 → 4 haploid spermatids w/ separated sister chromatids

• maturation (cell differentiation) of 4 spermatids → 4 haploid sperm cells

What structural changes do spermatids undergo to become mature sperm cells?

tail formation and mid piece full of mitochondria for energy

Oogenesis

gametogenesis in females

• germ-line cell from ovary undergoes mitosis → oocyte (diploid precursor)

• oocyte undergoes meiosis 1 → secondary oocyte + small polar body; uneven cytoplasm division

• secondary oocyte + small polar body undergo meiosis 2 → 4 haploid cells; 1 egg and 3 polar bodies

• egg cell undergoes differentiation to support fertilization + early development

Why is the cytoplasm divided unequally?

egg in larger portion of cytoplasm will be prioritized for nutrient intake; eggs in smaller cytoplasm are polar bodies that will degenerate in the future

Purpose of polar bodies

allow for chromosome and chromatin reduction without retaining excess cells

Meiosis 1 purpose

separate homologous chromosomes; not sister chromatids

pro metaphase in meiosis vs mitosis

defined in mitosis; not defined in meiosis

Five substages of prophase 1

(1) leptotene: chromatin condensation → visible chromosomes

(2) Zygotene: synaptonemal complex + tetrads/bivalents formation

(3) Pachytene: crossing-over of genetic material

(4) Diplotene: Chiasmata (crossover sites) become visible

(5) Diakinesis: nuclear envelope breakdown + spindle formation

Cohesins

part of synaptonemal complex; proteins that hold sister chromatids together along their lengths

components of synatonemal complex

lateral elements: on each homolog

central elements: in btwn

synaptonemal complex function

brings homologous chromosomes together for precision in crossing over of non-sister chromatids (step of pachytene)

How are DNA breaks in chromosomes repaired in synapsis?

paternal chromatid joined to segment of maternal chromatid and vise versa

Chiasma

site of crossover where genetic material is exchanged; essential for proper chromosome alignment and separation in meiosis 1

In metaphase 1, kinetochores of sister chromatids attach to the ___ pole

same; different poles in meiosis 2

Shugoshin role in anaphase 1

protects centromeric cohesins from degradation by separate

What happens to condensed chromosomes after meiosis 1 is complete?

they remain condensed

How does meiosis 2 resemble mitosis?

same goal; separate sister chromatids

Interphase of meiosis 1

typically nonexistent; no DNA replication reoccurs

separase function meiosis 2 anaphase

cleaves remaining cohesins at centromeres

Prophase: mitosis vs meiosis 1

mitosis: chromosomes condense but do not pair up

meiosis 1: homologous chromosomes pair up to form tetrads and cross over

Metaphase: Mitosis vs Meiosis 1

mitosis: individual chromosomes (dyads) line up at plate

meiosis 1: tetrads line up at plate and held together by chiasmata

Anaphase: mitosis vs meiosis 1

mitosis: sister chromatids separate

meiosis 1: homologous chromosomes separate while sister chromatids stay together (protected by shugoshin)

Outcome: mitosis vs meiosis (final)

Mitosis: two diploid daughter cells, genetically identical

Meiosis: four haploid cells w/ genetic disctinction

two main causes for genetic variability in meiosis

(1) crossing over of genetic info at prophase

(2) random alignment/orientation of chromosomes at center in metaphase

independent assortment outcome of both maternal chromosomes going to one side

daughter cells have only maternal or only paternal chromosomes

independent assortment outcome of one maternal and one paternal chromosome on each side

daughter cells are a mix; after meiosis 2, 4 genetically distinct haploid cells produced

which part of cell division does crossing over occur

prophase of meiosis 1, only

What triggers crossing over?

presence of double-strand breaks on chromosomes

what determines where a DNA break on chromosomes occurs?

Hot/cold spots;

• hot spots: loosely packed chromatin, likely to undergo recombination

• cold spots: tightly packed chromatin, unlikely to undergo recombination

number of chromatids in 46 chromosomes (2n)

92; 2 chromatids/chromosome

Meiosis 1 nondisjunction

homologous chromosomes don’t separate (both homologs migrate to same pole) → all 4 gametes abnormal; 2 with n+1 (24) and 2 with n-1 (22) chromosomes

meiosis 2 nondisjunction

sister chromatids don’t separate → 50% abnormal; 1 with n+1 (24) and 1 with n-1 (22), 2 remain normal (n or 23)

how does timing of nondisjunction impact gametes

the earlier, the greater the impact

Why are females more prone to nondisjunction?

prolonged meiotic arrest, twice:

• after prophase 1 (diplotene) from fetal development → ovulation

• atter meiosis 1 until fertilization triggers meiosis 2

possible outcomes (aneuploidies) of fertilization of abnormal gametes

monosomy (2n - 1); only one chromosome of a pair

trisomy (2n + 1); three copies of a chromosome

what happens to aneuploidy embryos?

most die, some survive (ex. trisomy 21)

trisomy 21

down syndrome; results from maternal nondisjunction of meiosis 1 → extra 21st chromosome