TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6
Is the ability of an organism to produce an individual of its type in order to increase the number of individuals of that species.
MEANS OR TYPES OF REPRODUCTION
Asexual reproduction is the type of reproduction which does not involve the fusion of gametes.
The lower species lies only on mutations as the main form of diversification and adaptation e.g. viruses and bacteria that reproduce asexually survive in harsh environment e.g. antibiotic resistant bacteria
and HIV viruses that mutate when patients do not take a combination of drugs. Mutations may be occurring slowly and hence the organism may fail to cope with the environmental conditions.
Sexually reproducing organisms on the other hand combination of some of variation (mutation, gene recombination, random alignment of chromosomes at metaphase I and the subsequent movement,
crossing over) which ensure diversity in their species and hence survival value.
TYPES OF ASEXUAL REPRODUCTION
Is the repeated division of cells to form more than two daughter cells e.g. Plasmodium which has infected liver cells.
E.g. Yeast, Hydra.
Budding is the form of asexual reproduction in which new individual is produced as an outgrowth (bud) of the parent and later is released as independent identical copy of the parent.
Is a form of asexual reproduction by which organism breaks into two or more parts each of which grows into a new individual.
Is the form of asexual reproduction which involve production of spores which are then dispersed for germination to grow into a new individual e.g. fungi, plants.
Is the form of asexual reproduction in which a bud grows and develop into a new plant.
Eg. A stem of cassava develops into a cassava plant.
Sexual reproduction is the production of new organism by combining the genetic material of two sex cells (gametes) from either a single parent or two different parents.
The two main processes of sexual reproduction are:-
PROPERTIES OF SEXUAL REPRODUCTION
The sex cells can be isogametes (i.e. gametes of same morphology from lower animals to higher animals respectively.
6. The process is affected by age where the young and old cannot reproduce while adults can.
ADVANTAGES OF SEXUAL REPRODUCTION
DISADVANTAGES OF SEXUAL REPRODUCTION
Even in internal fertilization so many risk of missing a mate, killing of sperms by acidic fluids in female reproductive organs etc reduces the probability of occurance of sexual reproduction.
This delays their production of offspring and may lead to extinction of the species in case of disaster.
Despite the setback sexual reproduction is the primary method of reproduction for the vast majority of microscopic organisms including almost all animals and plants. This is the most preferred type of reproduction because it allows the population to change (evolve) rapidly in response to a changing environment through recombination of alleles which makes organism vary.
Meiosis is the type of nuclear division which result with four daughters cucle each havinghalf the number of chromosomes of the parent cell. It is also termed as REDCTION DIVISION as it reduces the number of chromosomes from diploid (2n) to haploid (n). it is mainly a means of gametes formation …………………….
The human gamete has 23 chromosomes, 33 of which are autosomes and 1 sex chromosome of sparm cell has 23 chromosome
i.e. 22A + 1 sex chromosome
22A + x or y sex chromosome
22A + 1 sex chromosome
22A + x chromosome
SOME TERMS USED:
CHROMOSOME: A thread like structure visible in the nucleus of a cell during nuclear division
SEX CHROMOSOE: Chromosomes responsible for determination of sex of an individual
AUTOSOME: Chromosome responsible for determination of characters other than sex
Fig .Structure of Chromosome
PHASES OF MEIOSIS:
Meiosis is a long process as it passes through two cycles to completion
(a) Meiosis I or first meiotic division
(b) Meiosis II or second meiotic division
This reduces the number of chromosomes to half.
Meiosis I has the following phases:
(i) Interphase I
(ii) Prophase I
(iii) Metaphase I
(iv) Anaphase I
(v) Telophase I
(i) INTERPHASE I
It is a preparatory phase during which the nucleus is about to start dividing. The events of interphases one include the following:
(a) Replication of organalles
(b) Increase in size of the cell
(c) Replication of most of DNA and histories
(d) The chromosomes replicate so that each of them exist as a pair of chromotids being joined together by the centromere
(e) The chromosomal material will but no structure is clearly visible except the nucleoli
(ii) POOPHASE I
This is the largest of all stages. It is often described in five stages consecutive stages namely:
(c) Pachytene (lezypadid)
(a) LEPTOTENE (thin tread stage)
Leptetene stage initiates meiosis. During this stage:
(i) Chromosomes appear as unioiled thread like
(ii) Chromosomes appear to be longitudinally single
(iii) Chromosomes appear to have dense granules which occur at irregular intervals along their lengths. These are called choromosomeres
(b) ZYGOTENE (pairing stage)
This is initiated by the movement of chromones in the zygotene stage.
(i) Homologous chromosomes more close to one another and up they lie side by side, chromosomere by chromosomere under the influence of attraction force called SYNAPTIC FORCE
(ii) Synapsis begins at one or more points along the chromosome and unites along the entire length
(c) PACHYTENE (Thickening stage)
(i) Chromosomes are thickened and shorted by coiling and become visible
(ii) The nucleolus is attached to particular chromosomes
(iii) The synaptic force of attraction start to lapse and homologous chromosomes start to separate from each other.
Each chromosome appears a double structure.
(d) DIPLOTENE (Duplication stage)
(i) There is a complete duplication of each chromosome to produce two chromatids, thus each bivalent has four chromatids
(ii) The chromatids of homologous chromosomes cross over one another. At the point called chiasmata or cros over the number of chiasmata to be formed depends on the length of the chromosome.
At the chiasmata, chromosomes breaks and rejoin, thereby exchanging hereditary materials. As a result, genes from materinal chromosomes exchange with genes from paternal chromosomes leading to new gone combination in the resulting chromatids. This is a means of bringing about variation
(e) DIAKINESIS (Moving apart stages)
(i) The nucleolus detaches from its special bivalent and disappears.
(ii) The chiasmata tend to lose their original position and move towards the ends of chromosomes
(iii) The bivalent become considerably more contructed
(iv) Chromotids if homologous chromosomes continue to repel
(v) The contriolas of present migrate to the poles
(vi) The nuclear membrane starts to disintegrate and spindle fibre start to form
During Metaphase of meiosis
(i) The bivalent are arranged across the equitorial plate of the spindle with each centromere equidistant from the equatorial plate
(ii) The nuclear membrane has broken down completely
(iii) The spindle fibre forms and hold the centromers at the equator
During this phase:
(i) The two centromenes of each bivalent do not divide, instead the siste chromatids separate
(ii) The centomere pairs move toward the opposite poles
(iii) The chiasmata contents completely breakdown
(iv) The chromosome are separate into two haploid sets of chromosomes in the daughter cells.
This marks the end of meiosis. During this phase,
(i) The homologous chromosomes arrive at the opposite poles
(ii) Spindle fibre disappears, chromatids uncoil and the nuclear membrane rejoin around each pole
(iii) Cytoplasm dividing to form two daughter cells
N.B: In many plant cells there is no telephase, cell wall formation on interphase I. the cell pass straight from anaphase to prophase II.
This occurs in animal cell only is there is no interphase II in plant cell. Replication of DNA doe snot occur and energy stores of the cell increases. This stage is followed by meiosis II. The behavior of chromosomes in meiosis II is the same as that in meiosis I.
During this phase:
(i) Nucleoli and nuclear membrane start to disintegrate
(ii) The chromatids shorter and thicken
(iii) Cantoriols if present move to opposite poles the cell
(iv) Spindle fibre appear
The chentromeres align at the equator of the spindle
(i) The centromeres divide
(ii) The spindle fibres shorters and pull the centromeres to opposite poles
(iii) The eytoplasm started to cleave
– The chromosomes uncoil
– Spindle fibre disappear
– nuclear membrane reforms followed by complete cytokinesis
– Four daughter cells are formed each with half the number of chromosomes of the parent cell