Genbio Meiosis

Genetic diversity

• ppl w/ same parents don’t look the same bcs they’re made of diff sperm and egg cells

• important for meiosis cells to be diff bcs of this and for evolution

• no diff traits/genetic diversity = no evolution

Evolution

• goal is survival

• happens by chance

• not abt change and adaptation

• favorable mutations survive by chance bcs they’re adaptable to the current environment/situation

Mutations

can be good or bad, harmful or beneficial

Alleles

• diff forms/versions of the same gene

• new alleles arise by mutation

alleles example

• gene is hair color, alternative alleles of this gene are brown, red, black, blonde, etc hair

• blood type

Meiosis main goal

to not only make sperm and egg cells but also to encourage diversity and the conception of future offspring

meiosis

• process of chromosomal reduction (diploid cell is reduced to form haploid cells)

• haploid cells formed from this are gametes (2 main types: human egg and human sperm)

• 2 divisions (Meiosis I and Meiosis II)

• 4 daughter cells produced always

human egg

ovum

human sperm

spermatozoa

Fertilization

• “putting it all together”

• marriage/combination/contact/union/ of sperm and egg cells

• Haploid sperm + haploid egg = diploid zygote

• zygote created -> embryo -> fetus

• fraternal (different) or identical twins

Identical twins

same egg & sperm, divided into 2 embryo

Fraternal (different)

Left & Right ovary released two egg cell

fertilization components

need to be haploid to have the right # of chromosomes for the end result

Phases of meiosis (+ Interphase)

Interphase I -> Meiosis I -> Interphase II / Interkinesis -> Meiosis II -> cytokinesis

Interphase I

happens b4 meiosis I

same as mitosis interphase

Meiosis I

Prophase I

Metaphase I

Anaphase I

Telophase I

Prophase I

longest meiosis phase

1st: condensation of chromatin -> chromosomes

2nd: synapsis -

3rd: crossing over of genetic material between synapsed chromosomes

5 subphases

synapsis

full sister chromosomes look for partners w/ similar size and genetic content

pairing of homologous chromosomes [23 pairs for human chromosomes] -> connected together

end products are Tetrads/Bivalents

crossing over

exchange of bits of DNA of the Tetrad

Prophase I subphases

Leptotene

Zygotene

Pachytene

Diplotene

Diakinesis

Leptotene

chromatin condenses -> visible chromosomes

Zygotene

chromosomes line up to form homologous pairs (bivalents/tetrads) -> synapsis when the pairs join; synaptonemal complex forms

synaptonemal complex

like glue

binds chromosomes so the Tetrads stay in position as crossing over occurs

Pachytene

crossing over

some parts of the chromosomes overlap -> the chromosomal pairs exchange bits of DNA

chiasmata (where the chromosomes overlap/touch) form where these exchanges/crossing over occurred

Diplotene

synaptonemal complex dissolves and tetrads/chromosomes begin to separate from each other

Diakinesis

homologous chromosomes begin to separate further but’re still joined by a chiasmata

nuclear envelope and nucleoli disintegrate

meiotic spindle begins to form

microtubules attach to the chromosomes at the kinetochore of each sister chromatid

Metaphase I

chromosomal pairs line up along the cell’s equator

centrioles are now at opposite poles

Anaphase I

chromosomal pairs pulled apart by meiotic spindle

the sister chromatids stay together

Telophase I

microtubules disintegrate and a new nuclear membrane forms around each haploid set of chromosomes

chromosomes uncoil to form chromatin again and cytokinesis occurs after, forming 2 non-identical daughter cells w/ 23 chromosomes each

Cell size and content should be correct b4 entering meiosis II

Interphase II

• aka Interkinesis (interphase + cytokinesis)

• no DNA replication, only growth to produce 2 more cells

Meiosis II

same concept as mitosis

Prophase II

Metaphase II

Anaphase II

Telophase II

Prophase II

• 2 daughter cells w/ 23 chromosomes (46 chromatids) each simultaneously undergo meiosis II

• chromatin condenses -> visible chromosomes again

• nuclear envelope/membrane and nucleolus disintegrate

• spindle fibers begin to appear

• no synapsis and crossing-over

Metaphase II

the chromosomes (pair of sister chromatids) line up end-to-end along the cell’s equator

centrioles are now at opposite poles

meiotic spindle fibers at each pole of the cell attach to each of the sister chromatids

Anaphase II

sister chromatids separated into chromatids (that are considered individual chromosomes, 23 chromosomes/chromatid for each cell) and are pulled to opposite poles due to the meiotic spindle

Telophase II

• chromatids have reached opposing poles

• spindle disintegrates and the chromosomes recoil -> chromatin

• nuclear envelope forms around each haploid chromosome set -> cytokinesis: forming 4 identical haploid daughter cells w/ 23 chromosomes/chromatids each

Gametogenesis

• formation of gametes or sex cells from germ cells

• Mitosis (to create diploid cells) in initial stages to increase # of primordial germ cells -> meiosis (to reduce chromosome #) to produce haploid gametes (mature sperm or egg cells)

• happens in the gonads (testes in males, ovaries in females)

• two processes: spermatogenesis and oogenesis

Spermatogenesis

• formation of sperm cells by meiosis in the testes of males

• primordial germ cell (spermatogonium (singular) or spermatogonia (plural)) -> mitosis to divide identically (starts at 46 chromosomes) -> meiosis I -> 2 daughter cells w/ 23 chromosomes each -> meiosis II -> 4 daughter cells w/ 23 chromosomes each

• spermatogonia undergo mitosis IP b4 sexual maturity (puberty; indicated by testosterone production) and go into mitosis sometimes

• only starts at puberty

• testosterone spurs mitosis

• mitosis and meiosis still occur at old age

Oogenesis

process of forming an ovum (egg) by meiosis in the ovaries (specialized gonads)

starts during the 1st 2 weeks of the fetal stage → oogonium/oogonia develops in the womb of the fetus’s mother -> IP -> Prophase I then pauses at diplotene (can no longer create new oogonium; all are created at fetal stage) → diplotene continues when puberty is reached

egg cells decline w/ age

only 1 egg cell is produced and 3 polar bodies (cell’s that’ll disintegrate and get recycled) -> due to an egg cell’s large size

reason why egg cells decline w/ age bcs only 1 cell is produced per month

fetal stage

6-7 million eggs

upon birth

300,000 eggs

30-50y/o

300-400 eggs

25-28y/o

peak age to have a child

40-45y/o

high risk pregnancy bcs menopause soon

Meiosis errors

change in chromosome #

only happen due to faulty checkpts

chromosomal aberrations

chromosomal aberrations

1 missing or extra chromosome, or only half of a chromosome is present

Trisomy

Turner syndrome

Trisomy

state where humans have an extra autosome or sex chromosome due to non-disjunction

Down syndrome, patau syndrome, klinefelter syndrome

Down syndrome

Trisomy 21

due to an extra copy of chromosome 21

Patau syndrome

Trisomy 13

due to an extra copy of chromosome 13

high mortality rate bcs of brain and heart complications

Klinefelter syndrome

(47, XXY)

occurs when males have an extra X chromosome (47, XXY)

biological males w/ extra X are a bit more feminine looking

main complication is non-development of genitalia (infertile)

Turner syndrome

(45, X0)

deletion

aka X-syndrome

affects only females

not fully developed ovaries (infertile)

one of the X chromosomes (sex chromosomes) is missing or partially missing

46 Human chromosomes = 23 pairs

1st-22nd pair

23rd pair

Karyotyping

1st-22nd pair

autosomes (body chromosomes)

auto means body

mutations in this show in the body

23rd pair

sex chromosomes (X/Y chromosomes)

determine biological sex

XX (female), XY (male)

no YY bcs mother can only give and produce X chromosomes → father determines the biological sex of the child

weaker sperm cells carry X chromosomes

Karyotyping

arrangement of chromosomes based on their #

pairing and ordering all chromosomes of an organism