Sexual Reproduction

Reproduction with two parental inputs

Remember…

Asexual reproduction requires one parent, who reproduces offspring that are genetically identical to itself.

Some examples of asexual reproduction:

• Binary fission• Budding• Binary fusion• Rhizomes

Asexual vs. sexual reproduction

Asexual Sexual

Requires one parent Requires two parental inputs

Offspring are all clones

Offspring are genetically different

Quick process Slower process

Sexual reproduction

Sexual reproduction is the production of offspring of two parents using gametes.

A gamete is HAPLOID, while the resulting offspring are DIPLOID.

Haploid: Has only one set of chromosomes (23 in a human, including one sex chromosome)

Diploid: Has two sets of chromosomes (46 in human, including two sex chromosomes)

Gametes

In mammals, the gametes involved in sexual reproduction are the sperm cell and the egg cell.

Each has 23 chromosomes, including a sex chromosome (X or Y). They fuse together to form a single cell with 46 chromosomes, including two sex chromosomes (XX or XY)

Meiosis

Meiosis is the form of cell division that forms gametes.

Steps, in brief:• DNA replication• Division one• Division two• Four daughter cells are formed

Meiosis cont…

The cell that divides is DIPLOID (also known as 2n), and all the daughter cells are HAPLOID (also known as n)

Sometimes parts of the chromosomes are rearranged during meiosis, which gives rise to new combinations of genes (called GENETIC RECOMBINATION). This makes it very unlikely you will ever be genetically identical to anyone

Mitosis vs. Meiosis (an extra division)

Glossary words so far…

Gamete, meiosis, diploid, haploid

Heterogamy

The size of the gametes in animals is different. This is called HETEROGAMY

The female gamete (egg) is large, while the male gamete (sperm) is much smaller

Female gamete

Male gamete

Fertilisation

Fertilisation

Fertilisation is the moment the two gametes fuse together to create one diploid cell, called a ZYGOTE

This cell then divides over and over by mitosis to create a bundle of cells, then distinct organs and structures and finally a complete baby

The controversy

There is continued debate as to the point in which a zygote becomes a FETUS, and whether the change into a fetus is the beginning of life, or whether life only begins at birth.

This debate is especially significant politically, in the discussion of legal and moral abortion rights

External fertilisation

Some animals release their gametes into the environment, so fertilisation occurs outside the body

Sperm need a watery environment to swim to the eggs, so this is limited to aquatic environments

Examples of external fertilisation

Fish and amphibians (frogs) use a similar mechanism.

Eggs are jellylike and are released and fertilised in the environment.

Coral polyps release eggs and sperm into the environment and they mix and look like a cloud

Internal fertilisation

This occurs WITHIN the breeding female.

Can occur in marine environment (eg. Sharks, octopi), or on land (eg. Mammals, reptiles, insects, birds)

Some examples of internal fertilisation

Insects have genital structures at the end of their abdomen

Reptiles were the first to evolve a penis, for easy delivery of sperm to the female. They were also the first to have an egg with a protective covering

The male bird does not have a penis, but sperm is deposited in the female via close genital contact. An egg forms inside the female, forms a hard shell and is laid

Most mammals retain the fertilised egg within the female, where the offspring grows. Monotremes (echidna and platypus) lay fertilised eggs

Glossary words

Heterogamy, ova, ovules, sperm, zygote, fetus, external fertilisation, internal fertilisation

The benefits of sex

Mixing genetic pools means that poor genes (such as disease genes) can be “bred out” of a population

This mixing also means that good genes (eg. That increase strength) can be “selected for”, and increase in the population

Mixing also allows for ADAPTATION to environments

Are there any drawbacks to sexual reproduction?

Effort required to find a suitable mate is considerable

Only half of your genetic information is passed on to the next generation, thus diluting your own contribution to the species as time goes on.

Glossary

Adaptation, genetic recombination, mixing genetic pools.

Life cycles

Different organisms have different life cycles

Human Life Cycle

Generalised plant and animal Life Cycles

Life cycles of plants

Some plant life cycles (eg. Ferns) have two stages:

• The normal diploid stage of the main plant (SPOROPHYTE)

• The plant develops parts (GAMETOPHYTE) where both male and female gametes (in most plant species), which fuse together inside the flower/other plant structure. This creates the seed, which will give rise to the seedling.

Sexual structures of flowering plants

Sex in flowering plants

Most plants have both pollen (male) and egg (female) producing organs. This means that they can SELF POLLINATE, if needed.

In self pollination, pollen falls into ovary• BUT – no mixing of the gene pool

They also may be fertilised by other flowers’ pollen via movement from wind or insects

Sex in flowering plants cont.

Some flowers do not make both pollen and eggs. They must be fertilised by a different flower.

Sometimes male and female flowers exist on the same plant, and can fertilise each other. This is called MONOECIOUS. Eg. pumpkin

Sometimes the male and female flowers grow on different plants. They are DIOECIOUS. Eg. kiwifruit

Glossary

Life cycle, sporophyte, gametophyte, stigma, ovary, stamen, monoecious, dioecious, self pollination.

Determining sex in animals

Firstly; an important distinction: SEX is biological, while GENDER is social

If an animal can produce both eggs and sperm, they are called HERMAPHRODITES. We often call humans with indeterminate sex this as well, but it’s incorrect.

Simultaneous Hermaphrodites

These animals are producing both sperm and eggs at the same time.

Examples: snails, earthworms

Sequential Hermaphrodites

These animals have the potential to produce either sperm or eggs, dependent on the situation.

For example, some species of Angelfish live in “harems” of many females with a single mating male. If that male dies or becomes infertile for any reason, one of the large females turns into a male.

Some anemone fish do the same, but start out as large groups of males with one female

Determining sex in mammals In mammals, sex is determined by the

presence of the Y chromosome. If the Y chromosome is present, the animal is MALE.

This causes some problems occasionally when some people are born with an extra chromosome. If this extra chromosome makes their sex XXY, the person may show some characteristics of both males and females, but they are legally male because of the presence of the Y chromosome.

Sex chromosomes

Mammals have an XX/XY sex chromosome system, but other animals have some slightly different set ups

• Bees: males are haploid, while females are diploid

• Birds: ZW is female, while ZZ is male• Cockroaches: females have XX, but

males have XO, which means they have only one sex chromosomes and it is X