In this section we’re going to talk about human reproduction. It’s a pretty big topic so it’s been broken down into: sexual reproduction, female reproduction, male reproduction, fertilisation and DNA.
All living things reproduce by either asexual reproduction which is the production of offspring without the use of gametes or sexual reproduction (how humans reproduce) by producing offspring by the joining of sex cells.
In sexual reproduction the offspring will have genes from two parents. This is unlike asexual reproduction where the offspring are essentially a clone of one parent.
One of the advantages of sexual reproduction is that offspring are able to adapt to the changing environment. Because there is room for genetic variation in sexual reproduction there is the chance for offspring to adapt well to survive changing conditions.
This variation in genes appeals to the theory of natural selection which was first put forward by Charles Darwin. Darwin proposed that offspring (not just from humans) would face selection pressures such as competition for food and mates and the majority would die, whilst the few who were better adapted for survival would mate together and pass on their fit genes to their offspring.
Over generations this process ensures that the genes best suited for survival in any given habitat are maintained in a population whilst the unfit genes are lost as the majority of offspring die before reaching reproductive age.
For humans to reproduce an egg from a female must be fertilised by the sperm of a male. So let’s take a closer look at each of the different biological systems that lead up to fertilisation.
The ovaries produce the female gametes called ova. Once a month, an egg is released from an ovary and slowly travels down the oviduct, this is also called the fallopian tube. During sexual intercourse sperm are deposited in the vagina and they swim up through the cervix, though the uterus and along the oviduct, where they are most likely to encounter an ovum.
During puberty, both males and females go through changes brought about by the actions of the hormone oestrogen (primarily in females) and testosterone (primarily in males).
Oestrogen causes female hips to widen, breasts to develop, underarm and pubic hair to grow and the development of a sex drive.
Females will also start their menstrual cycle, or periods as they are more commonly referred to.
After a female undergoes her first menstruation she becomes sexually mature and capable of having offspring. The menstrual cycle lasts approximately 28 days with specific events occurring on specific days.
Between days one to four menstruation occurs the endometrium lining of uterus is shed away.
On day four the ovaries begin to release oestrogen, which causes an egg to mature in a follicle in (usually) one of the ovaries. The oestrogen also helps to rebuild the uterus wall that was broken down during menstruation.
On day 14 there is a surge of oestrogen, causing the mature egg to be released from the follicle into the oviduct. The remainder of the follicle stays inside the ovary and is called a corpus luteum.
Between days 14-28 the corpus luteum releases progesterone, which maintains the endometrium of the uterus in case of fertilisation and implantation of an embryo.
Finally on day 28 if fertilisation does not occur then the corpus luteum slowly breaks down and progesterone levels drop, which causes the endometrium to be shed again (menstruation).
However, if the egg is fertilised then the corpus luteum is preserved and progesterone levels remain high, which keeps the endometrium thick and prevents menstruation.
And that’s everything you need to know about the female reproductive system, so onto male reproduction.
During puberty males change by the action of the hormone testosterone. Testosterone causes male voices to deepen, the penis to grow, shoulders to broaden, pubic, underarm and facial hair to grow, aggression to increase and the development of a sex drive.
The testes produce the male gametes called sperm cells. During sexual intercourse, the sperm travel through the sperm tube towards the prostate gland. On its journey the prostate gland and the seminal vesicles add fluids to the sperm creating a liquid called semen.
Once the sperm tube merges with the tube from the bladder, the combined tube is then known as the urethra, and semen travels down the urethra along the penis and is ejaculated into the vagina during the male orgasm.
If the sperm reaches an egg and it becomes fertilised, it is now known as a zygote.
We know that during sexual reproduction, both parents must create a sex cell (gamete), which contains half of their DNA. Then one gamete from each parent must meet and fuse together in a process called fertilisation.
This can happen internally inside the female (e.g. in humans) or externally (e.g. in frogs). The fertilised egg is known as a zygote and this undergoes cell division to become an embryo.
Once the embryo develops further and becomes recognisable as a particular species, it is known as a foetus.
And that’s basically human reproduction; but if we take a closer look we can see what is happening in the DNA, genes and chromosomes before and after a zygote is made.
To produce female offspring there must be an XX combination of chromosomes, and to produce a male there must be an XY combination.
A human egg contains 23 chromosomes. In a haploid cell there is only one copy of a chromosome, but in a diploid cell there are two copies.
A diploid cell, like a skin or blood cell, has the full amount of DNA, whereas a haploid cell only contains half the full amount of DNA.
The function of DNA is to code for protein molecules which can then be used for many functions in the body. The precise order of the bases in a gene controls the precise way the protein is synthesised from amino acids. A slight change in DNA base order will change the structure of the protein being synthesised from that gene, and hence change the function of that protein.
Deoxyribonucleic acid (DNA) consists of three repeating units: a ribose sugar, nitrogenous base and a phosphate group. The sugar and phosphate form the uprights of the ladder of DNA, with the bases forming the steps.
There are four possible bases: guanine (G), cytosine (C), thymine (T) and adenine (A). In a double strand of DNA it is the bases which hydrogen bond to each other to form a double helix.
A always bonds to T, and C always bonds onto G. There are 46 chromosomes in a diploid human cell, containing an estimated total of 20,000-30,000 genes, and therefore billions of bases.
The order of bases in DNA can appear random, but is highly conserved across individuals. For example, 99.9% of your DNA will have the same order of bases through it as the person sat next to you.
Alternative forms of genes are called alleles. An allele that is always expressed as a phenotype is dominant. If an allele is only expressed when the dominant one is not present that is called recessive.
An individual with two copies of the same allele is called homozygous. The range of alleles an individual has is called their genotype.
An individual’s phenotype describes the effect a person’s genetic make-up has on their appearance.