Home ADVANCED LEVEL TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

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TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6
TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

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.

Sexual reproduction.

ASEXUAL REPRODUCTION

Asexual reproduction is the type of reproduction which does not involve the fusion of gametes.

CHARACTERISTICS.

Proceeds without fusion of gametes (asexual).

A single parent is capable of asexual reproduction.

It is a product of mitosis.

It occurs fast enough to prevent the chances of occurring of sexual reproduction.

It has few stages before the offspring are produced.

ADVANTAGES

It is a quick process yielding a substantial number of offspring to increase the chances of survival of the species during unfavourable environmental conditions.

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

It eliminates the possibilities of occurrence of sexual reproduction.

No changes of genetic makeup as the process is a product of mitosis. This is a way of maintaining good qualities in a population.

No mixing of materials from more than one parent therefore contamination and infections are minimized.

DISADVANTAGES

Fast yielding of offspring leads to overcrowding and hence competition over necessities of life among organisms e.g. light, food, mineral salts, air etc.

The DNA replication does not mutate it produces daughter cell exactly the same as the mother cell no variation. This makes the individual less potential evolve into new species and cope with the

environmental challenges.

It may be a way to propagate defective gene into species/progeny the defective gene may affect the entire population.

The process involves no mixing of gene from the two different parents this eliminates the diversity or divergence among the individuals of the same species hence limits the advancement of the entirespecies.

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

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

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

Binary fission

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

Multiple fission

Is the repeated division of cells to form more than two daughter cells e.g. Plasmodium which has infected liver cells.

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

Budding

E.g. Yeast, Hydra.

Definition:

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.

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

Fragmentation E.g. spirogyra, ribbon, worms.

Is a form of asexual reproduction by which organism breaks into two or more parts each of which grows into a new individual.

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

Sporulation

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.

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

Vegetative propagation

Is the form of asexual reproduction in which a bud grows and develop into a new plant.

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

Eg. A stem of cassava develops into a cassava plant.

SEXUAL REPRODUCTION.

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:-

Meiosis which involves halving the number of chromosomes.

Fertilization involving the fusion of two gametes and the restoration of the original number of chromosomes.

During meiosis the chromosomes of each pair usually cross over to achieve homologous recombination during sexual fertilization.

PROPERTIES OF SEXUAL REPRODUCTION

It involves the application of gametes (sex cells) hence termed sexual reproduction. The two sex cells may come from two different parents (i.e. dioecious) or from single parent (i.e. monoeciousness).

The sex cells can be isogametes (i.e. gametes of same morphology from lower animals to higher animals respectively.

The organisms carrying out sexual reproduction may be monoecious or dioecious.

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

The process involves lots of stages which may delay the product.

The process is associated with lots of risks e.g. risk to miss a mate, to miss fertilization etc.

Provides variation among the offsprings by its;

Meiosis involving crossing over which produce, recombinant chromosomes and normal chromosomes.

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

Random fertilization where genes are randomly mixed (i.e. random combination of genes occurs due to any sperm fertilize the given egg).

6. The process is affected by age where the young and old cannot reproduce while adults can.

ADVANTAGES OF SEXUAL REPRODUCTION

It involves more space for genetic shuffling which leads to the evolution of the organism.

It produces variation in the offspring when crossing over during prophase I and random assortment during metaphaseI occurs. Variation will increase the survival of the species and prevent from extinction.

The process delays the production of offspring due to age factor. The delayed product is a natural way to reduce over population hence less competition among organisms.

DISADVANTAGES OF SEXUAL REPRODUCTION

Very uncertain especially for external fertilization where the sperm has to meet the ovum outside, this might not occur; the process of fertilization might fail on the way to produce the zygote.

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

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.

Maturity of offspring is slowly achieved.

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.

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

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:

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

An ovam:

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

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

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

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

MEIOSIS I

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

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

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:

(a) Laptotene

(b) Zygotene

(c) Pachytene (lezypadid)

(d) Diplotene

(e) Diakinesis

(a) LEPTOTENE (thin tread stage)

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

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

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

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

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

(ii) Synapsis begins at one or more points along the chromosome and unites along the entire length

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

(c) PACHYTENE (Thickening stage)

During pachytene

(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

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

 



TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

(e) DIAKINESIS (Moving apart stages)
During Diakinesis:
(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
Study problem:

TOPIC 3 : REPRODUCTION (I) ~ BIOLOGY FORM 6

Describe the events of prophase of meiosis I and comment on biological consequences of chiasmata formation.

MATAPHASE I

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

ANAPHASE I

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.

TELOPHASE I

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.

INTERPHASE 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.

PROPHASE II

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

METAPHANE II

The chentromeres align at the equator of the spindle

ANAPHASE II

(i) The centromeres divide

(ii) The spindle fibres shorters and pull the centromeres to opposite poles

(iii) The eytoplasm started to cleave

TELOPHASE II

– 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

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