Mitosis & Meiosis - Information management in cell divison

how do Prokaryotes separate their DNA during cell division?

Simple process where a single DNA copy is separated as the cell itself divides.

Why is Eukaryotes DNA separation more complicated?

The genome is divided into multiple chromosomes, requiring organized process like Mitosis and the involvement of spindle fibers.

• exact copy of information is passed to the next cell.

• 2 parent cells split in half and combine information

What is the purpose of DNA separation during cell division?

To produce exact copies of DNA for daughter cells.

What is a karyotype?

The complete set of chromosomes as seen during cell division. It is used to assess chromosomal abnormalities and variations in chromosome number.

How are these chromosomes/karyotypes identified?

By their size, shape, and banding patterns during metaphase as well as the centromere location.

• Number

• Size

• Shape

• Staining

• Patterns

Human karyotype chromosomes (Should know the differences)

metacentric, submetacentric, acrocentric, telocentric

note: the location of the centromere is what makes them visually different.

Metacentric

centromeres are located near the middle of the chromosomes which results in 2 nearly equal arms. (examples 1, 3, 16, 20)

Submetacentric

centromere is slightly off-center, resulting in one chromosome arm being noticeably longer than the others. (2, 4, 5, 6, 7, 8, 9, 10, 11, 12, 17, 18, X)

Acrocentric

centromere is located near the end of the chromosome, rather than the center. (13, 14, 15, 21, 22)

Telocentric

Considered unstable and arise from chromosome breakage near the centromere. Should be eliminated during cell division.

note: healthy human karyotypes do not contain any telocentric chromosomes.

What are homologous chromosomes?

Chromosome with the same or similar genes but different origins.1 from each parent. They pair during meiosis and can recombine genetic material.

• 22 pairs of autosomal chromosomes

• 1 pair of sex chromosomes

What does Allele mean?

one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.

Banding patterns

gemsa dye (G-bands)

dark - inactive region

bright - active region

p arm

short - petite

q arm

long - french for tail

What is the haploid number (n)?

one member of a homologous pair.

• example: gametes

  • reproductive cells

• humans n=23

What does diploid (2n) mean?

A cell with homologous pairs of chromosomes, one from each parent.

• somatic cells

  • any cell of a living organisms other than the reproductive cells

• humans n=23

What are sister chromatids?

exact copies of a chromosome made during DNA synthesis/replication.

What are the main phrases of the cell cycle?

Interphase (G1, S, G2) and M phase (Mitosis and Cytokinesis)

What happens during the G1 of interphase?

Cell growth and preparation for DNA replication. During G1, the cell increases in size, produces RNA, and synthesizes proteins necessary for DNA replication.

What does G0 mean?

This means that it is in the resting phase of the cell cycle, where the cell is not actively preparing to divide and may exit the cell cycle temporarily or permanently.

What happens during the S phase of interphase?

DNA synthesis and replication occurs, resulting in the duplication of chromosomes. The cell prepares for mitosis by ensuring that each chromosome is copied accurately.

What happens during the G2 of interphase?

The cell prepares for mitosis and undergoes checkpoints. During G2, the cell continues to grow, produces proteins, and checks for DNA damage, ensuring that all chromosomes have been accurately replicated before mitosis begins.

What happens during the M phase?

Mitosis

What are the stages of mitosis?

Prophase, prometaphase, metaphase, anaphase, telophase

What happens during prophase?

Chromosomes condense, spindle microtubules form, and centrosomes separate.

What happens during Metaphase?

Chromosomes will align in the center between the two spindle poles. With each sister chromatid attached to opposite centrosomes.

• “metaphase plate”

  • imaginary line that runs through the center of a cell during cell division. The chromosomes line up before separating.

What is a centrosome?

Involved in cell division. Before cell division it duplicates and once division begins the two centrosomes mow to opposite ends of the cell.

What happens during Anaphase?

Sister chromatids separate to opposite poles due to kinetochore activity and and microtubule disassembly at the ends. The poles move apart.

What is a centromere?

Region on a chromosome that acts as the attachment point for spindle fibers during cell division. It holds the two sister chromatids together and ensures that they are properly separated and pulled apart to opposite ends of the dividing cell. It aids in allowing equal distribution of genetic material to daughter cells during mitosis and meiosis.

What is kinetochores?

Protein complex that assembles on the centromere of a chromosomes. Acts as an attachment point for spindle microtubles during cell division.

why do sister chromatids stick together until anaphase?

molecular staples/glues

What are sister chromatids held together by? What breaks it down?

Cohesin holds them together in metaphase until separase digests it just before anaphase.

What happens during telophase?

Chromosomes reach the ends of the cell and decondense. Nuclear envelope reforms and spindle fibers disperse.

note: cytokinesis can be simultaneous with telophase.

What is cytokinesis?

The physical separation/division of the cytoplasm into two daughter cells.

• animal cells

  • contractile ring pinches cells into two halves

• plant cells

  • cell plate forms dividing cell into two halves

Mitosis

• product: two smaller but genetically identical cells

• conservative process for growing more cells

  • growth of organism

  • repair

  • replace cells

• asexual progeny

  • progeny is an exact copy of parent “clone”

What happens if cytokinesis is skipped?

a multinucleated cell forms.

What happens if the M phase (Mitosis) or anaphase is skipped?

The nucleus may contain more than 2n chromosomes. (multiple copies of genome).

What is the result of mitosis mistakes? Early in development.

Leads to the formation of mosaic organisms, where different cells have different genetic compositions.

• blaschko’s lines

  • skin cell lineages

  • these are usually invisible but with this mistake, it causes the skin lines to show up.

Meiosis

chromosome separation during gamete formation.

• allows eukaryotic fair and equal exchange

• produces haploid gametes

  • each cell is haploid with one homologous chromosome

  • each gamete has a unique combination

why make haploid gametes?

• maintain a constant amount of genetic material between generations

• to create diversity

  • two parents make different combinations of progeny

What is the difference between meiosis and mitosis?

Mitosis is a cell division that produces two identical daughter cells with the same number of chromosomes as the parent cell. While meiosis produces 4 genetically unique daughter cells, each with half the number of chromosomes as the parent cell.

What is the purpose of meiosis?

To create haploid gametes by separating homologous chromosomes. Which ensure genetic diversity.

How does Meiosis contribute to genetic diversity?

Through independent assortment of different homologous chromosomes and crossing over makes sister chromosomes different.

What must happen before meiosis begins?

The cell must undergo interphase, where sister chromatids are formed in the S phase.

What happens during prophase 1 that is unique to meiosis?

synaptonemal complex - physical pairing of homologous chromosomes.

chiasmata - result of recombination, holding homologous chromatids together.

crossing over of homologous chromosomes - makes the sister chromatids no longer idential.

What is recombination?

Genetic material is exchanged between homologous chromosomes during prophase 1 of meiosis. Results in new combinations of alleles and increasing genetic diversity in offspring. Ex: chiasmata

• diversity from segregation of homologous chromosomes & recombination

What is the significance of crossing over in prophase 1?

It increases genetic diversity by allowing exchange of genetic material between homologous chromosomes.

What is interkinesis?

A short resting phase between meiosis 1 and 2, with no DNA replication.

What happens in Meiosis 2?

Sister chromatids are separated, similar to mitosis, resulting in haploid gametes.

What are two key mechanisms that create genetic diversity in gametes?

1. independent assortment of homologous chromosomes

2. crossing over during prophase 1, making sister chromatids non-identical.

What proteins are involved in chromosome attachment and separation?

1. cohesin: holds sister chromatids together

2. separase: digests cohesin to allow chromatid separation

3. shugoshin: protects cohesin at centromeres during meiosis 1 to keep sister chromatids together.

What is mitotic nondisjunction, when can it occur?

failure of chromosomes to separate in anaphase 1 or 2. It leads to gametes with n+1 and n-1 chromosomes (aneuploidy). affects only the daughter cells of the dividing cell causing mosaicism within an organism.

• mistake in mitosis

• meiosis 1

  • 2 gametes n + 1

  • 2 gametes n - 1

• meiosis 2

  • 2 gametes normal

  • 1 gamete n + 1

  • 1 gamete n - 1

what is parthenogenesis?

A form of reproduction without fertilization, where a 2N cell is created from an egg alone due to failure or reversal of meiosis.

• change in meiosis where meiosis 2 is skipped or reversed

Meiotic noondisjunction

when chromosomes fail to separate properly during meiosis. results in gametes with an incorrect number of chromosomes. When fertilized results in genetic disorders due to aneuploid.

• down syndrome

  • extra copy chromosome 21

• patau syndrome

  • extra copy chromosome 13

• edwards syndrome

  • extra copy chromosome 18

• lots of variations with X or Y

What is somatic mosaicism? What does i t cause?

different genetic populations of cells exist within the same individual. within the brain cells it causes overgrowth.

bacterial conjugation

genetic material is transferred between two bacterial cells through direct contact. HGT. creates new combinations

What are somatic cells?

any cell of a living organism that is not a reproductive cell

what are germline cells?

the reproductive cells that carry genetic information passed from one generation to the next.

Describe spermatogenesis using genetics.

diploid spermatogonial stem cells in the testes undergo multiple rounds of mitosis to proliferate. It is followed by meiosis to produce haploid spermatids which then differentiate into mature spermatogenesis results in each sperm carrying a single unique set of chromosomes ready for fertilization.

1. spermatogonia 2n reside in the seminiferous tubules and divide by mitosis to produce more.

2. some differentiate into primary spermatocytes 2n after DNA replication of s phase.

3. homologous chromosomes separate, forming two haploid secondary spermatocytes n.

4. each secondary spermatocyte contains sister chromatids still attached at the centromere.

5. sister chromatids separate causing 4 haploid spermatids n that are genetically unique:

   1. independent assortment during meiosis 1

   2. crossing over during prophase 1.

6. spermatids undergo change to become mature spermatozoa

   1. condensation of chromatin - streamlined DNA transport

   2. acrosome - enzyme cap for egg penetration

   3. flagellum - motility

   4. sheds excess cytoplasm

Describe oogenesis using genetics.

Diploid oogonium in a female ovary undergoes meiosis to produce a single haploid ovum, accompanied by the formation of polar bodies. Effectively reducing the chromosome number by half to prepare for fertilization with a sperm cell.

1. diploid oogonium 2n undergoes mitotic divisions to produce oogonia.

2. oogonia enter the S phase and duplicate their chromosomes forming primary oocytes (2n), each with sister chromatids. - DNA replication

3. its arrested in prophase 1 due to oocyte maturation inhibitors and remain here until puberty.

4. FSH stimulates primary oocytes to resume meiosis

5. homologous chromosomes separate producing 2 unequal haploid daughter cells

   1. secondary oocyte n

   2. first polar body n

6. then its arrested in metaphase 2 until fertilization.

7. lf fertilized secondary oocyte completes meiosis 2 and makes

   1. ovum n - mature haploid egg

   2. second polar body n - degenerates

8. the ovums nucleus fuses with the sperms nucleus to restore the diploid chromosome number 2n

meiosis 1 vs meiosis 2

1. first division in meiosis when homologous chromosomes separate.

   1. reduces the chromosome number by half

   2. reductional division

2. second division where sister chromatids separate

   • 4 haploid daughter cells with the same chromosome number as the cells that entered meiosis 2.

   • equational division

meiosis 1: prophase 1

longest/complex phase

• humans

  • mitotic prophase (shorter)

  • male meiotic prophase 1 (days)

  • female meiotic prophase 1 (years)

after fertilization with a normal gamete

trisomy

• 1 extra chromosome 2n + 1

monosomy

• 1 fewer 2n - 1