Sexual Reproduction and Meiosis Flashcards
Somatic cells: dip or hap?
Diploid (each contain 46 chromosomes)
How many cells are somatic in multicellular organism?
-almost all of the cells
Diploid cells:
-two copies/sets of each chromosome (2n) inherited from each parents
DO NOT contribute
to inheritance in the next generation
in humans each diploid cells contains 46 chromosomes
22 paires of homologous autosomes + 2 sex
Originated by mitosis so..
chromosome number is maintained in mitosis
Autosome chromosomes
Human diploid cells have 22 homologous autosomes and a pair of SEX CHROMOSOMES
Homologous autosomal chromosomes
Pairs of chromosomes (not sex chromosomes)
Homologous:
having the same organisation, relative position, value and structure
In humans: 22 copies of autosomes
numbered in descent based on their size
Homologous pairs contain…
The same kind of genes at specific positions (locus), but slightly different sequences (two alleles, variant versions of genes)
Sex chromosomes
Pairs of chromosomes that participate in sex/gender determination at birth
What are the types of sex chromosomes?
X and Y
Which pair number are the chromosomes X and Y
23rd pair of chromosomes
Females
XX in their cells
Males
XY in their cells
Similar?
differ from autosomes in form, size and behaviour
Sex cells are also called
GAMETES
Gametes
reproductive cells of the organisms = EGG and SPERMATOZOON involved in sex reproduction
Haploid or diploid: gametes
haploid - contain only one set of chromosomes
in humans…
23 chromosomes= 22autosomes + sex chromosome
originated by diploid germ line cells by meiosis
chromosome number is divided in half by meiosis
contribute to inheritance to the…
next generation
Sexual reproduction steps
3 steps
1)
Spermatozoa and egg (n) are produced by meiosis from diploid germ-line cells (2n)
- maternal and parental chromosome sets are partitioned into the single chromosome sets of the gametes
2)
Fertilisation (fusion) of gametes regenerates the diploid structure of the somatic cells (zygote with homologous chromosomes from both)
3)
The zygote develops into a new individual, through many rounds of mitosis
Meiosis KEY features:
> used for sexual reproduction
- created gametes, carrier only a single set of chromosomes (n)
- occurs in specialised germ-line cells that reside in the ovaries or testes
A cell that produces 4 nonidentical (genetically dissimilar)
Daughter cells having half the number of chromosomes of the parents cell (2n»_space;> n)
process of duplication
2n ? 4n > 2n + 2n > n + n + n + n
Meiosis broken down:
- one round of DNA replication
- followed by two rounds of nuclear division (Meiosis I and Meiosis II)
- Meiosis v Mitosis
Used only for sexual reproduction
v
Used for asexual reproduction and growth
- Meiosis v Mitosis
Requires two nuclear divisions (Meiosis I & II)
v
Requires one nuclear division
- Meiosis v Mitosis
Halves chromosome number (2n to n)
v
Preserves chromosome number (2n to 2n)
- Meiosis v Mitosis
Produces four daughter nuclei
v
Produces two daughter nuclei
- Meiosis v Mitosis = daughter cells
Produces daughter cells genetically different from parent and each other
v
Produces daughter cells genetically identical to parent and to each other
Meiotic interphase
3 main events:
- interphase
- meiosis I
- meiosis II
Interphase
constituting in 3 phases: G1, S , G2
In S phase, DNA is replicated (sister chromatids)
Meiosis I
> (1st division), with 4 stages + cytokinesis I
Separation of the pairs of homologous chromosomes, after the process of pairing of the duplicated homologous chromosomes along their entire length
Homologous chromosomes are similar but not identical
Meiosis II
> (2nd division) with four stages + cytokinesis II
Separation of the sister chromatids (copies of the same chromosome)
Meiosis : PMAT(C) I
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
- Cytokinesis I
Meiosis II : PMAT(C) II
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
- Cytokinesis II
Meiosis I - reductional division. Task?
Separation of homologous chromosomes
Prophase I
- The longest and most peculiar stage of the whole meiotic process
- The meiotic spindle also forms from the poles of the cell (centrosomes)
-Nuclear envelope breaks down
Duplicated chromsomes homologs (+ sex chromosomes) are..
condensed and brought together (lined up) during a process called PAIRING (or synapse)
What is the structure of pairing called?
BIVALENT
- give rise to a unit of four sister chromatids stuck together (tetrad)
Crossing over (in prophase I)
pairing of homologous chromosomes is coupled with homologous recombination
What is the importance of crossing-over?
Genetic exchange between DNA segments of the non-sister chromatids within tetrads of homologous chromosomes
How does crossing-over work?
- the chromatids break in the same place and sections of chromosomes (alleles) are swapped (based on similar sequences)
Can cross over happen on more than one occasion per tetrad?
One or more cross-over events can occur per tetrad at sites called chiasmata (s. chiasma)
How does crossing-over contribute to genetic variation?
> Allele reassortment
This creates chromosomes that are unique mosaics (mixture) of the maternal and paternal homologs from which they arise
Offspring will have different set of alleles to their parents
Does crossing-over appear in MITOSIS?
NO - only in meiosis
2) Metaphase I
