Week 8 sexual reproduction and meiosis Flashcards
Somatic cells (Diploid cells)
Multicellular organisms are produced by rounds of mitosis from a single cell
The majority of cells are SOMATIC (“body” cells, other than sex cells/ gametes)
Autosome chromosomes
Human diploid cells have 22 homologous autosomes and 2 sex chromosomes
Homologous (pairs) autosomal (not sex chromosomes) chromosomes
->Homologous (pairs) as they have the same size and shape with the same organisation
->They contain the same kind of genes at specific positions (locus), but with slightly different
sequences (two alleles, variant versions of genes)
Sex chromosomes
Sex/gender determination at birth
In humans, there are X and Y (23rd pair of chromosomes)
Females have two X chromosomes in their cells
Males have both a X and a Y chromosome
X chromosome is larger than Y and they carry different genes
Karyotype
A visual representation of an individual’s complete set of chromosomes, including their number, size, and shape
Sex cells (gametes)– Haploid cells
Multicellular organisms produce sex cells or gametes
Gametes -> reproductive cells (egg cells and spermatozoa), involved in sex reproduction
Haploid cells: containing only one set of chromosomes (n)
Originated by diploid germ line cells by meiosis chromosome no. is divided in half in meiosis
->Contribute to inheritance to the next generation
Sexual Reproduction (stage 1)
1)Spermatozoa and egg (n) are produced by meiosis from diploid germ-line cells (2n)
maternal and paternal chromosome sets are partitioned into gametes
Sexual Reproduction (stage 2)
2)Fertilisation -> fusion of gametes regenerates the diploid structure of the somatic cells (zygote – 2n)
Sexual Reproduction (stage 3)
The zygote develops into a new organism (mitosis rounds)
Meiosis – Key features
Used only for sexual reproduction
Producing gametes, sex cells carrying only a single set of chromosomes (n)
->It occurs from specialised germ-line cells that reside in the ovaries or testes
Producing 4 non-identical (genetically dissimilar) daughter cells having half the number of chromosomes of the parent cell (2n -> n)
Involves one DNA replication and two nuclear divisions (Meiosis I and II)
Meiosis overview
Three main events;
1)Interphase, constituting in 3 phases: G1, S , G2 ->In S phase, DNA is replicated (sister chromatids)
2)Meiosis I (1st division), with 4 stages + cytokinesis I
->Separation of the pairs of homologous chromosomes
->Homologous chromosomes are not identical (paternally- and maternally-derived alleles)
3)Meiosis II (2nd division) with four stages + cytokinesis II
->Separation of the sister chromatids (copies of the same chromosome)
Meiosis 1
Separation of homologous chromosomes;
1)Prophase 1
2)Metaphase 1
3)Anaphase 1
4)Telophase1
5)Cytokinesis 1
Prophase 1
The pairing of homologous chromosomes is coupled with cross-over
Genetic exchange between DNA segments of the non-sister chromatids within tetrads of
homologous chromosomes
Prophase 1 cross over
-The chromatids break in the same place and sections of chromosomes are swapped
-Allowing alleles recombination (maternal & paternal)
-One or more cross-over events can occur per tetrad
->Cross-over as a major source
of genetic variation;
-Allele reassortment
-Creating chromosomes with unique combination of maternal & paternal alleles
-Offspring will have different set of alleles
Cross-over does NOT occur in mitosis
Metaphase 1
The spindle microtubules from both poles capture each tetrad (through kinetochores), and move them towards the centre of the meiotic plate
Homologous chromosomes are aligned on either side of the meiotic spindle -> the orientation is random (independent assortment)
Metaphase/anaphase 1checkpoint
To control the correct chromosome attachment
and alignment
Anaphase 1
-Synapsis breaks up
-Homologous chromosomes separate from each other (1 tetrads to 2 dyads)
-Each (with two chromatids) moves towards opposite poles
Telophase 1
Nuclei are formed around the separated chromosomes
Cytokinesis 1
2 daughter cells are formed having one set of chromosomes (1n, haploids), but duplicated
Daughter cells are genetically different with a unique combination of maternal and paternal chromosomes
Interkinesis
A short interphase before Meiosis II
->There is NO replication of DNA
Meiosis II
Sister chromatid separation
Producing 4 gametes, haploid daughter cells
Errors in meiosis
-Errors can occur (Non-disjunction events) ->wrong
separation of homologous chromosomes (meiosis I)
or sister chromatids (meiosis II)
-Gametes can lack a chromosome or have >1 copy
-Upon fertilisation, such gametes can form embryos, with
unbalanced genetic assets -> genetic diseases
Down syndrome or Trisomy 21
A disorder with cognitive and physical disabilities
->Caused by an extra copy of chromosome 21
Genetic Variation of Meiosis: Significance
Meiosis and sexual reproduction result in genetic diversity
Independent assortment of chromosomes in meiosis
Genetic reassortment of crossing-over in meiosis
Random fertilisation of gametes from different individuals
Genetic variability is essential for a species’ survival and adaptation to changing environments
