Meiosis Flashcards
Diploid cell
- (2n)
- Cells contain 2 complete sets of each type of chromosome
- One set originated from the father and one from the mother
Haploid cell
- (n)
- cells containing a single set of unpaired chromosomes
Meiosis
- One diploid cell (2n) divides
- To form 4 haploid cells (n)
- chromosome number reduced to half
- Production of gametes (or gametophyte generation in plants)
Alternation of generations
Alternation between diploid and haploid phases
Stages of meiosis
Meiosis 1:
- Prophase 1;
- Metaphase 1;
- Anaphase 1;
- Telophase 1
Meiosis 2:
- Prophase 2;
- Metaphase 2;
- Anaphase 2;
- Telophase 2
Homologous chromosomes
- Chromosomes containing the same gene sets
- Similar size, shape and information
- One paternal and the other maternal
One duplicated chromosome at metaphase can be labelled with…
- Two sister chromatids
- joined together at the centromere
DNA replication takes place during
Interphase (S-phase)
Meiosis Prophase 1
- Nuclear membrane breaks down
- Chromosomes condense
- Bivalents form
- Crossing over occurs
- Centrioles produce spindle
Meiosis Metaphase 1
- Homologous chromosomes align along equator
- Independent assortment of chromosomes
- Spindle fully formed
- Diagrams show evidence chiasmata
Meiosis Anaphase 1
- Homologous chromosomes move to opposite poles
- Spindle threads contract pulling chromosomes
- Diagrams show evidence chiasmata
Meiosis Telophase 1
- Each pole has haploid (n) chromosomes
- Nuclear membrane reforms
- Spindle disappears
- Cytokinesis usually visible
Meiosis Prophase 2
- Nuclear membrane disappears
- Chromosomes condense
- Spindle apparatus forms
Meiosis Metaphase 2
- Single chromosomes line up at equator
- Spindle fully formed
Meiosis Anaphase 2
- Sister chromatids split at centromere into two chromosomes
- Spindle contracts pulling single copy chromosomes to opposite poles
Meiosis Telophase 2
- Nuclear membranes reform
- Spindle disappears
- Cytokinesis occurs
- Four haploid daughter cells produced
Purpose of meiosis
- In animals: production of gametes (n)
- In plants: production of spores/gametophyte generation (n)
Gametogenesis and what it is called in males and females
- Production of gametes
- In males: spermatogenesis
- In females: oogenesis
Alternation of generations
When organisms alternate between haploid (n) and diploid (2n) forms during their life cycles
Groups that have an alternation of generations
- Protists
- Fungi
- Plants:
- Bryophytes
- Pteridophytes
- Gymnosperms and
- angiosperms
The different life forms of mosses and ferns
- Gametophyte (n) produces gametes (n)
- Gametes fuse to form zygote (2n)
- Zygote grows into sporophyte (2n)
- Sporophyte undergoes meiosis to produce spores (n)
- Spores grow into gametophyte
Ploidy
The chromosome number in an organism
The importance of meiosis
- Chromosome number halved - to maintain ploidy when gametes fuse
- Cells produced after meiosis genetically unique - introduces and mixes up variation of genes in a species
How variation is ensured in offspring through meiosis
- Random independent assortment of chromosomes and segregation
- Chiasma formation during crossing over
- Random gametes involved in fertilisation
Term used when a problem occurs during meiosis result in a new chromosome number
Aneuploidy (a-new-ploidy)
Nondisjunction of chromosomes
- When a chromosome pair(s) fails to separate during meiosis
- resulting in aneuploidy for the gametes formed
Polyploidy
- When cells have more than two complete sets of chromosomes.
- More common in plants.
Down’s syndrome
- Person with a ploidy of 47
- Extra chromosome number 21
- Also called trisomy 21
Causes of Down’s syndrome
- An extra chromosome 21
- Possibly greater risk from external factors e.g. Radiation
- Risk increases with maternal age
Medical conditions associated with Down’s syndrome
- Developmental difficulties
- Hearing and vision defects
- Heart abnormalities
- Leukemia
- Thyroid disorders
- Dementia in later life
- Shorter lifespan
Similarities between mitosis and meiosis
- DNA replicates prior during interphase
- Nuclear membrane and nucleolus break down during prophase
- Spindle forms during prophase
- Chromosomes condense during prophase
- Chromosomes line up on equator during metaphase
- Spindle contracts during anaphase
- Cytokinesis follows both meiosis and mitosis
Different locations in mitosis vs meiosis
- Mitosis: all somatic cells and asexual reproduction
- Meiosis: reproductive organs (in humans: testes and ovaries)
Process differences between mitosis and meiosis
- Mitosis both haploid or diploid cells divide; meiosis only diploid
- Mitosis has one division of chromosomes; meiosis separation in meiosis 1 and division in meiosis 2
- Mitosis has no pairing of homologous chromosomes; meiosis pairing occurs
- Mitosis one nuclear division; meiosis has two
- Mitosis has only one cytokinesis event; meiosis has two
Differences in the cells produced between mitosis and meiosis
- Mitosis produces two cells; meiosis four
- Mitosis maintains chromosome number; meiosis halves it
- Mitosis daughter cells with same genes; meiosis with unique genes
- Mitosis daughter cells identical; meiosis unique
Functional differences between mitosis and meiosis
Mitosis:
- growth,
- repair and
- asexual reproduction
Meiosis:
- for gamete formation,
- to maintain chromosome number in life cycle
