DNA, Genes and chromosomes

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DNA, Chromosome, Gene

A cell contains a nucleus, the nucleus contains, chromosomes, chromosomes are long strands of DNA coiled up, a gene is a short section of DNA.

Human bodies are diploid, diploid means they have two copies of each chromosomes, arranged in pairs. A human nucleus contains a total of 46 chromosomes, this means a human diploid number is 46.

Genes are the instruction book for organisms, a long list of instructions of how an organism is put together and made to work.

Each gene in a DNA molecule is a separate code or chemical instruction for a particular protein.

Proteins are important in the body as the control most processes, they are also responsible for determining inherited characteristics e;g eye colour and blood type.

By controlling the production of proteins, genes also control our inherited characteristics

There are different versions of the same gene, which will give different versions of a characteristic, like blue or drown eyes. The different version of the same gene is called an allele or alleles.

Double helix

A DNA molecule has two strands coiled together in two spirals called a double helix.

The strands are held together by chemical bases. Four such chemical bases are adenine (A), cytosine (C), guanine (G) and thymine (T).

These bases are paired and will always pair in the same way, adenine to thymine and cytosine to guanine. This is complementary base-pairing.

DNA is a type of nucleic acid.

Reproduction

an organism can reproduce in two ways, asexually and sexually, in each way cell division takes place, cell division can happen by mitosis or meiosis.

Asexual reproduction.

Mitosis - asexual reproduction

An ordinary cell can reproduce to make a new cell, by simply dividing in two. Both cells are new cells, are genetically identical to the original cell, and contain the exact same genetic information

This type of cell division is call mitosis. Some organisms are able to produce offspring using mitosis, this is none as asexual reproduction, organisms that can asexually reproduce are bacteria and some plants.

A sexual reproduction involves, only one parent. The offspring have identical genes to the parent - so there's no variation between parent and offspring.

MITOSIS is when a cell reproduces itself by splitting to form two cells with identical sets of chromosomes

When diploid cells divide by mitosis you get two cells that are both diploid. If a cell gets a signal to divide, it first needs to replicate its DNA so that there is a copy for each new cell. The DNA forms X-shaped chromosomes, each "arm" of the chromosome is and exact duplicate of the other.

The chromosomes then line up in the centre of the cell, cell fibres then pull the chromosomes apart. The two arms of each chromosome go to opposite ends of the cell.

Membranes form around each of the sets of chromosomes. These become the nuclei of the two new cells. Lastly the cytoplasm divides.

You now have two new cells, containing exactly the same DNA and re genetically identical.

Mitosis for repair and growth

Mitosis doesn't just happen for asexual reproduction. It is how plants and animals grow and repair damaged tissue. Cloning also involves mitosis.

Sexual reproduction and Meiosis

Unlike asexual reproduction, sexual reproduction needs to have a set of parents, a mother parent and father parent, these two organisms supply combine genetic information to produce offspring which is genetically different to either parent.

In sexual reproduction the mother and father organisms produce gametes, a male gamete is a sperm, a female gamete is an egg cell.

Gametes are haploid, which means they only have half the number of chromosomes of a normal cell. In a human gametes have 23 chromosomes, so the haploid number is 23.

At fertilisation a male gamete and female gamete fuse together to form a zygote, a fertilised egg. The zygote ends up with the full set of chromosomes.

The zygote then undergoes cell division by mitosis to form an embryo, The embryo inherits characteristics from the mother parent and father parent as it has received chromosomes from both, the chromosomes decide how you will turn out.

The fertilisation of gametes is random, this is what causes genetic variation of the offspring.

Sexual reproduction involves the fusion of male and female gametes. Because there are two parents, the offspring contain a mixture of their parents genes.

Gametes produced by meiosis

In humans meiosis only takes place in the reproductive organs. It is different to mitosis as it doesn't produce identical cells.

MEIOSIS produces four haploid cells whose chromosomes are NOT identical.

Before the cell starts to divide, it duplicates its DNA so there is enough for each new cell. One arm of each chromosome is an exact copy.

In the first division in meiosis the chromosomes line up in pairs in the centre of the cell. The pairs are then pulled apart so the new cell only has one copy of each chromosome. Some from the mother chromosomes, some from the fathers chromosomes go into each of the new cells. Each new cell will have a mixture of mother and father chromosomes, creating genetic variation.

In the second division the chromosomes line up again in the centre of the cell, as with mitosis the chromosomes arms are pulled apart

You get four haploid gametes each only with a single set of chromosomes, the gametes are all genetically different.

Sexual reproduction in plants

Flowers contain both male and female gametes and have both male and female structures.

The stamen is the male reproductive part. The stamen consists of the anther and filament.

The anther containing the pollen grains, these produce the male gametes - sperm

The filament is the stalk that supports the anther.

The carpel is the female reproductive part. it consists of the stigma, style and ovary.

The stigma is the end part that the pollen grains attach too.

The style is a rod like section that supports the stigma.

The ovary contains the female gamete, the egg.

Pollination

This is the transfer of pollen to the stigma, this is so that the male gamete sperm can fertilise the female gamete egg in sexual reproduction.

Cross Pollination

This is the transfer of pollen from one plant to the stigma of another plant, cross pollination rely on insects or the wind to transfer the pollen to the stigma.

Adaptation for insect pollination

Plants adapted for insect pollination will have brightly coloured petals to attract insects.

They will also have scented flowers and nectaries, glands that secrete nectar.

They make big sticky pollen grains, these pollen grains stick to the insects as they go from plant to plant.

The stigma is also sticky, so any pollen on an insect will stick to it.

Adaptation for wind pollination

Plants adapted for wind pollination will, not need to have bright colourful petals to attract insects, the will not need to have a scent or nectaries as they will not need to attract insects.

They produce a lot of pollen grains, these grains are small and light so they can be carried on the wind.

They will have ling filaments, that hang the anther outside of the plant, so a lot of the pollen gets blown away by the wind.

They will have large feathery stigmas, which hang outside of the plant, to catch pollen that is blown past by the wind.

Fertilisation is the fusing of gametes

Once a pollen grain lands on a stigma, either with the help of an insect or the wind the process of fertilisation starts.

A pollen tube grows down from the stigma, through the style and into the ovary.

A nucleus from the male gamete, makes it's way down the pollen tube to join the female gamete in the ovary.

Fertilisation occurs when a male and female gamete fuse together to create a zygote.

The zygote then divided by mitosis to create an embryo.

Each fertilized gamete forms a seed and the ovary develops into a fruit around the seed.

Germination.

A seed will lay dormant for long periods of time, until conditions are right for germination.

Water will activate enzymes that break down the food reserves in the seed.

Oxygen provided for respiration to produce the energy for growth

Temperature needs to be suitable for the enzymes inside the seed to work, this is seed dependant.

A seed takes in water and starts to grow using its stored energy.

The first root starts to grow down in the soil

The shoot grows up

Finally, extra roots grow and the first green leaves appear.

Germinating seeds

A developed seeds contains an embryo and a food store wrapped in a hard seed coat.

When a seed starts to germinate it gets glucose from respiration from its own food store, this gives it the energy to grow.

Once the plant has grown enough and has developed green leaves, it can then get its energy from photosynthesis.

Asexual reproduction in plants

Some plants reproduce asexually, they can do this naturally, or with our help they can do it artificially, artificial asexual plant reproduction is called cloning.

Naturally

some plants can asexually reproduce, plants such as strawberry plants, send out runners, these runners grow along the soils surface sideways, at various points these runners will root and start to grow a new plant, these plants are clones of the parent plant and there is no genetic difference.

Artificially

For generations gardeners have been taking cuttings, by taking a cutting of a plant you can use it to make a genetically identical copy or clone. This is a quick and cheap method of reproducing good quality plants from a parent plant.

Human reproductive systems

The male gametes are the sperm, after puberty these are made in the testes all the time.

Sperm is mixed with a liquid to make semen, which is ejaculated from the penis into the vagina of the female during sexual intercourse

Urethra this carries the sperm through the penis during ejaculation. Urine also passes through the urethra to exit the body.

Erectile tissue swells when filled with blood to make the penis erect

Testis where sperm are made, Glands produce liquid that is added to sperm to make semen.

Vas deferens a sperm duct that carriers the sperm from the testis towards the urethra.

Scrotal sac hangs behind the penis and contains the testes.

The female reproductive system makes ova or eggs.

Female gametes are ova, an ovum is an egg, which is produced once every 28 days from one of the 2 ovaries.

It then passes into the fallopian tube, it is here where it might meet sperm from sexual intercourse.

If the ovum isn't fertilised it will break down and exit through the vagina.

If it is fertilised the ovum starts to divide and it travels down the fallopian tube to the uterus and attaches itself to the endometrium uterus lining, a fertilised ovum develops into an embryo.

Puberty and hormones

At puberty your body starts to release sex hormones. In men it is testosterone and in women it is oestrogen.

These hormones trigger your secondary sexual characteristics.

Oestrogen will: develop extra hair under the armpits and pubic area, widen the hips, develop breasts, start ovum release or periods.

Testosterone will: develop extra hair on the body and face, develop muscles, enlarge the penis and testicles, start sperm production and deepen the voice.

The menstrual cycle

starting puberty, leads to women staring the menstrual cycle, the cycle involves the body preparing to receive a fertilised egg in the uterus.

Stages:

There are 4 stages to the menstrual cycle.

Stage 1: Day 1 is when the bleeding starts, this is the uterus lining breaking down - normally about 4 days

Stage 2: The uterus lining begins to build up again, day 4- 14 a spongy layer full of blood vessels is produced ready to receive a fertilised ovum

Stage 3: An ovum develops and is released from the ovum, day 14.

Stage 4: The wall is maintained for about 14 days, if no fertilised ovum reaches the uterus by day 28, the uterus lining begins to break down again.

Control of the menstrual cycle

Two hormones are produced in the ovaries and maintain the cycle, oestrogen and progesterone.

Oestrogen will: Cause the uterus lining to grow and thicken, and at day 14 stimulate an ovum release, by way of luteinizing hormone LH

Progesterone will: Maintain the uterus wall lining, when the level of progesterone falls the uterus lining will begin to break down. If a fertilised egg reaches the uterus the progesterone levels will be maintained to support the uterus lining during a pregnancy.

Pregnancy and embryo

Once an Ovum has bee fertilised it develops into and embryo and plants itself to the uterus, when it develops further it becomes known as a fetus.

Placenta: Once the embryo has implanted on the uterus, the placenta develops, this allows the blood of the mother and the baby pass very close, this allows the exchange of food, oxygen and waste.

Amniotic fluid: The amnion membrane forms around the embryo and is full of amniotic fluid, this fluid protects the embryo from knocks and bumps.

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