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The Human Body
Reproduction Part I
Meeting in a crowd - the egg and sperm story
INTRODUCTION Species - fertilisation and characteristics
The reproductive system consists of sex organs that enable men and women to produce children
to continue the species. Male organs called testis make sperm. Female organs called ovaries
make eggs. Sperms and eggs unite to create babies. The process of uniting the two reproductive
cells is called fertilisation.
In man, animals and plant life cycles, all species need to reproduce, so that the species does not
die out through aging. The characteristics of each species like outward apearance of face, body
and limbs and the form and function of the internal system are different in these species. The
information on these differences are kept stored in the nucleus in the form of genes in the nucleus
of the egg and the sperm of that species.
The Genetic Code Instructions storehouse/mix - and match
Inside the nucleus there is a small molecule called DNA (dioxyribonulic acid) this carries the
instruction to make protines and all the characteristics of a human being.
The DNA molecule are in long thin strands, the strands are in pairs and two strands are joined by
cross links like the steps in a ladder. The strands are also coiled in a double spiral (double helix)
these strands are the chromosome and small sections of it are known as genes.
When egg and sperm unite (fertilisation), these characteristics in the two cells are pooled and the
characteristics of the spring will be from those received from both the parents.
How genes are passed on
Apart from the sex chromosomes, all other 44 come in pairs.
In every pair, both chromosomes give instructions for the same thing, the same features are
coded for by genes in the same place on each chromosome, called the gene locus. So you have
two alternative instructions for each feature.
Some chromosomes may carry many genes, called polygenes, to code for a single feature.
Most features are a mix of the two sets of instructions. But with some, there can be no alternative
- one or the other must win. The gene that wins is said to be 'dominant'; the loser is 'recessive.'
Genes Function How genes function
Building proteins
Proteins are built from different combinations of chemicals called amino acids. These are all
available in the cell, so to make a protein, the DNA simply has to instruct the cell as to which ones
to stitch together.
Codons
The bases in DNA are set in groups of three, called codons, and each spells out certain amino
acids.
There are 64 ways four bases can be ordered in groups of three.
RNA
DNA is too valuable to use directly as a mould. So when the cell needs a new protein, it sends an
enzyme to copy just the right bit on to a similar chemical called m(messenger) RNA
mRNA makes the new protein in a part of the cell called the ribosome by linking amino acids.
Heredity How you are similar
The young ofspring (you and your siblings) resemble your parents. This is because the instruction
in the DNA has been carried out and passed on to you. You will pass them on to your children.
Humans have 46 chromosomes in every cell except the sperm and the egg, these have 23
chromosomes, the 23rd chromosome is sex chromosome. The females carry XX chromosome
while the male carries an XY chromosome. The female egg will thus always have an X
chromosome while the male sperm may have an X or a Y chromosome. therefore the sperm will
decide if the ofspring will be a boy or girl. The other characteristics like colour of eyes, hair and
skin are also inherited through the other chromosomes.
The steps of reproduction are:
* Liberation of Ova from the ovary
* Entry of male sperm into the female reproductive system
* Fertilisation of the ova by the sperm
* Transport of fertilised ovum to the uterus
* Impantation of the ovum
* Development of placenta for nutrition of the forming embryo (baby)
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