OCR GCSE Biology- B5

What is a tissue?

A group of similar cells.

What is an organ?

A group of tissues. Groups of organs make systems within the organism.

What is mitosis?

The division of cells for growth, repair and replacing old tissues, to create new cells with identical chromosomes and genes as the parent cells. For this to take place, cells go through a cycle of growth and division, until the cell can no longer divide.

How does mitosis occur?

In the parent cell, each chromosome copies itself- the two strands of each DNA molecule separate, and new strands form alongside them. The copies are then pulled away from the original chromosome. When this happens, the cell can divide, creating two daughter cells with the same number of chromosomes as the parent.

What is meiosis?

The division of cells in the testes and ovaries, to create gametes with half the number of chromosomes as the parent cells. The gametes are sex cells, used for sexual reproduction.

How does meiosis occur?

In the parent cell, each chromosome copies itself- the two strands of each DNA molecule separate, and new strands form alongside them. The chromosomes separate, and move to opposite sides of the cell, along with their copies, giving an equal amount on both sides. The cell then divides, and the copies separate from the original chromosomes. The second cell division then happens, giving four gametes with half the number of chromosomes as the parent cell.

What is fertilisation?

A male gamete (sperm) and female gamete (egg) fuse together to form a single cell of a new individual- a zygote. As the two gametes have half the number of cells as the parent, the zygote has one whole set of chromosomes, in pairs- one coming from the mother and one from the father.

What is an embryo?

A cluster of cells, formed when a zygote divides by mitosis. The cells become specialised, and, eventually, a fully developed baby is produced.

What is variation?

The differences between parents and offspring, based on the combination of alleles passed on from parents. Offspring from the same parents can have different characteristics because of this.

What are genes?

Sections of DNA, present inside the chromosomes in a cell's nucleus, which control the organism's growth and development of characteristics.

How do genes control characteristics?

They provide the instructions for the production of proteins, which is based on the order of bases that hold together the section of DNA. This order is the genetic code for a particular protein.

What are the four bases?

Adenine (A) and thymine (T), which always pair together, and cytosine (C) and guanine (G), which always pair together.

Where does the production of proteins take place?

In the cytoplasm, outside of the nucleus, because the DNA is too large to leave the nucleus. The information from the genes therefore has to be transferred.

What are amino acids?

The substances that make up proteins, determined by the sequence of bases in a gene. A group of three pairs of bases, called a triplet code, codes for a single amino acid in a chain.

How is the protein produced?

The DNA is unraveled at the relevant gene, and the instructions, or code, are copied onto smaller molecules, called messenger RNA (mRNA). These can leave the nucleus, so they do, and carry the instructions into the cytoplasm. Here, it is taken to the ribosomes, which decode the triplet code. The amino acids made are then joined together to form a protein, or polypeptide.

What are embryonic stem cells?

The unspecialised cells in a human embryo that can have any gene switched on to become any kind of specialised cell. They can be found up to the 8 cell stage.

What happens in an embryo after the 8 cell stage?

The cells become specialised- genes inside them switch on, or become active- and can form different types of tissue. All the cells contain the same genes, but all have different genes active, as each individual cell only requires some to do their function.

How can embryonic stem cells be used?

They are completely unspecialised, so could be given any function, and used to treat diseases and disorders or to repair damaged tissue.

Where else can stem cells come from?

Umbilical cord blood and adult bone marrow, however neither of these would be completely unspecialised, so may not be as useful as embryonic stem cells.

What is therapeutic cloning?

A form of medical treatment, using stem cells from an embryo produced by cloning a person's cell. The person's immune system would not treat the cell as foreign, so it will have an effect and not get rejected. If the stem cell was from a different host, the immune system would reject it.

How does therapeutic cloning work?

An egg cell has its nucleus removed, and it is replaced with the nucleus of the patient's cell. The egg cell is then stimulated to divide by mitosis, as if it was a zygote. This starts to form an embryo, and, at the 8 cell stage, the stem cells are taken. The cells are then grown in a container with warm nutrients, and treated to develop properly into the cell type required.

What are meristems?

The areas in plants where growth occurs, because of unspecialised cells dividing by mitosis, differentiating and becoming specialised.

What are the two types of meristems?

Lateral, where increases in girth occur, and apical, where increases in height occur.

What are xylem tubes?

Plant tissue, made by changes in the hormonal conditions of the plant's environment, used to transport water and soluble mineral salts from the roots to the stem and leaves, and to replace water lost during photosynthesis and transpiration.

What are phloem tubes?

Plant tissue, made by changes in the hormonal conditions of the plant's environment, used to transport dissolved food to the whole plant for storage or respiration.

How can plant clones be made?

Cuttings are taken from a plant, and put in a rooting hormone. The roots of new plants start to form and new plants, genetically identical to the parent, develop.

How can new plants be made with desirable features?

The cuttings taken for cloning can contain unspecialised cells, which can be developed to give clones with desirable features.

What is auxin?

The main plant hormone used in horticulture, which affects cell division at the tip of a shoot and causes cells to grow just under the tip, allowing the stem or roots to grow longer.

What is phototropism?

Plants changing the direction they are growing in, based on the light source. Plants need light to survive, so they grow towards the light source.

How does phototropism work?

Auxin, having been produced in the shoot tip of a plant, moves down and causes cells further down to grow. When a light source shines on a shoot, the auxin nearest the light source is destroyed, so the auxin becomes concentrated on the side furthest away and the shoot bends towards the light.

What happens when a light source is directly above a plant?

The plant will grow straight up.

What happens when a light source is placed at an angle, and the shoot is covered in a transparent cap?

The plant will grow and bend towards the light.

What happens when a light source is placed at an angle, and the shoot is covered in an opaque material?

The plant will grow straight up, as if the light source isn't there.

How do plants grow?

The cells divide by mitosis, and specialise into the cells of roots, leaves and flowers. Plants continue to grow for their whole life, unlike animals.