B5 Flashcards Preview

Z- Biology 456 - GCSE > B5 > Flashcards

Flashcards in B5 Deck (27):

All the instructions of how a person grows and developes are stored in the ...


The DNA is coiled around the chromosomes in the nucleus of your bodily cells


What are DNA molecules

DNA molecules are series of genetic code which are basically long lists of instructions which are utilised by organelles in the cell in order to produce proteins


Describe the structure of DNA

DNA has a double helix structure , therefore there are two strand coiled together . Each strand is made of small bases called nucleotides , each nucleotide possesses a small molecule called a base , there are four different bases . The strands of DNA form along a sugar phosphate back bone


Bases of DNA

Adenine (A)

Cytosine (C)

Guanine (G)

Thymine (T)

The two strands of DNA are held together by the bases which always pair up in the same way

G and C

T and A

This is known as base pairing


What is a gene

A gene is a section of DNA which codes for a particular protein


Amino acids

Proteins are strings of amino acids .

Each different protein has a different combination of amino acids which are ordered differently in order to produce different proteins .

Each 3 bases on mRNA (every triplet base) codes for a single amino acid , hence the order of the bases ultimately decide the order of the amino acids.

It is the order of these amino acids which decide the protein .


The process of protein synthesis

Genetic code (DNA) is found in the nucleus of a cell, however the proteins the DNA provide instructions for , are not produced in this organelle but are rather produced in the cytoplasm of a cell

Proteins are produced by organelles called ribosomes

DNA which is stored in the nucleus can not leave the nucleus as the molecules are too big.

To move the instructions to the ribosome where they can be read and followed a copy of the DNA must be produced that can leave the nucleus

To do this the double helix of DNA unzips this means base pairs come appart and there is a template of bases .

Upon this template of bases a strand of messenger RNA is formed , this molecule is smaller than DNA , it is only a single strand and is much shorter hence can leave the nuclear membrane progress through the cytoplasm and arrive at the ribosome.

The mRNA arrives at the ribosome shortly after leaving the nucleus , the ribosome is an organelles that is located within the cytoplasm of a cell.

The job of a ribosome is to form chains of amino acids through reading the messenger RNA and connecting the corresponding amino acids to the triplet bases. This is done in a sequential order so the order of bases dictated the order of amino acids

As chains of amino acids are string together proteins begin to be formed , of are these proteins that are then utilised for the cell for reactions growth and development


What happens if the order of bases is changed

If a shingle base from the DNA is added or removed an entirely different protein can be produced this is a consequence of upsetting the triplet bases which code for amino acids , of amino acids are strung together in an altered order a different protein will be produced



Cell division by mitosis produces two new cells which are genetically identical to the parental cell. Cells need to divide in this way in order to allow a person to grow and develop and to replace damaged cells , cells dividing to do so is known as the cell cycle. There are two main parts of a cell cycle

Stage one ----> growth

In the fist stage of mitosis cells physically grow all their genetic information doubles as well as all of its organelles such as mitochondria , ribosomes etc. A bodily cell would ordinarily have 46 chromosomes but in this stage of growth prior to division the cell will contain 92 chromosomes as it will have two copies of each chromosome . In order to duplicate the DNA the DNA unzips , free floating nucleotides then pair up with the strands hence two additional strands are formed as both of the original strand are now paired with newly formed strands, these two strands of DNA are genetically identical to one another as the base painting is the same.

Stage 2 ----> division
Once the cell has doubled its contents it can begin its division . The two copies of the DNA coil around the chromosomes in such a way that each arm of the x shaped chromosomes are genetically identical copied of each other

These chromosomes line up along the equator of the cell and are pulled apart by spindle fibres which extends from opposite sides of the cells , this separates the two arms of the chromosomes.

New nuclear membranes then form around the two separated sets of chromosomes prior to the division of the cytoplasm which produces two new cells .

As the single cell which contained 92 chromosomes divided into 2 each of the two daughter cells produced contain half of this (46 chromosomes ) and hence the two bodily cells are the same as all the others in the body . The cells are also genetically identical to the parental cell.


What is uncontrolled mitosis




Meiosis is a type of cell division that produces gametes , gamers are cells with half the quantity of genetic information as the normal bodily cells (23 chromosomes), gametes are the sperm and the egg

During sexual reproduction the sperm cell and the egg cell combine to produce a gamete this combintation of the two gamete produces a cell which once again has 46 chromosomes this is an average bodily cells.

Meiosis involves two divisions where as mitosis only involves a single division.

Like mitosis the first step in meiosis is to replicate all of the genetic information in the cell so the cell contains 92 chromosomes. The copy of the chromosome is know as the sister chromitate . These chromosomes line up along the equator of the cell and are pulled appart by spindle fibres into two knew cells.

There is then a second division within the two knew cells , again the chromosomes line up along the equator of the cell and are pulled appart by spindle fibres however this time the arms that are pulled appart are not genetically identical to one another thus the cells produced in this division are genetically different from and from the parental cell .

The second division produces a further two cells and as there are two cells undergoing the second division 4 daughter cells are produced all o which are gametes as they only contain 23 chromosomes as a consistency of the cell division of a cell containing 46 chromosomes.

When a zygote formed from the gamete of a mother and a father it will then have a combination of maternal and paternal chromosomes nance have different maternal and paternal alleles . A human cell needs to have 46 chromosomes to function this the union of the two gametes containing 23 chromosomes produces the zygote



gametes are produced by meiosis however as soon as the gametes have combined to produce a zygote cell division by mitosis begins to produce embryonic stem cells followed by specialised cells , tissues organisms etc


Cross breeds

Some cross breeds of animals may not contain an even number of chromosomes per cell hence are inferior as their cells cannot divide by Meiosis to produce successful gametes


Unspecialised cells that occur after the process of fertilisation are known as

Embryonic stem cells


Embryonic stem

Embryonic stem cells are undifferentiated this means they are able to divide to produce any type of cell , for instance a red blood cell or a nerve cell as prior to the 8 cell stage they are undifferentiated


Specialisation // stem cells

Cells in a multicellular organisms are not all one generic type of cell they are all specialised for different purposes.

Groups of specialised cells are called tissues

Groups of tissues are called organs

In humans cells are unspecialised until the 8 cell stage

The process of cells becoming specialised is known as differentiation

Adults also have stem cells in their bone marrow but there are not as Versatile as embryonic stem cells and cannot specialise into as wide of a variation of things


Capacity of cells

Although all genes contain the same genetic information , not all the genes writhing a cell are active . Genes can be switched on and off during. Specialisation to dictate what instructions the cell will require , those that the cell does not need are switched off hence only some cells produce particular proteins . Embryonic stem cells are a blank canvas , in specialisation genes have a wide range of potential and can be switched on or off to produce an elaborate range of different specialised cells. Adult stem cells however are harder to work with as t is difficult to switch on and off genes which have already been selected


Replacing damaged tissues

Adult stem cells are already used to treat some people with diseases for instance people with sickle cell anaemia are given bone marrow transplants so that new blood cells can be produced to replace the faulty old ones

Embryonic stem cells can be extracted from very early humans embryos : these embryonic stem cells can then be stimulated to specialise into cetain types of specialised cells which could be grown in a lab to replace old faulty cells for instance heart tissue could be produced to replace damaged heart tissue in people with heart disease . To control the differentiation scientists control the the conditions around the cell in order to stimulate a specific set of genes to be switched on or off. This is quite complex and difficult hence research is necessary in the field of science .

Some people believe it is unethical to use embryonic stem cells as this stem cells could have grown to be a human life but instead are destroyed / jeopardised in research . As a consequence research into embryonic stem cells is regulate by the government .



The first step in cleaning is to take an embryonic stem cell and remove its genetic material , this genetic material is discarded of

Next a nucleus extracted form an adult cell is implanted into the empty egg cell.

Under the right conditions genes in the adult nucleus can be switched back on or turned off normally through electrical stimulation this helps the embryo form.

The clone of the donor can then have the embryonic stem cells extracted , these cells can then be controlled in their specialisation to produce replacement cells.

In exam board terms

I'm carefully controlled conditions of mammalian cloning it is possible to reactivate (switch on ) in the nucleus of a body cell to form cells of all tissue types.


Mitosis of plants

Like animals plants also divide by mitosis to produce new cells.

In plants only cells within special regions called meristems are mitotically active. Meristems are found in the tips of roots and shoots , new cells are produced in these regions that are unspecialised hence can specialised to produce any type of plant cell this share qualities with embryonic stem cells.

Unlike embryonic meristems stem cells can continue to be produced in the meristems for as long as the plant lives not simply up to an 8 cell stage . The meristem cells can specialise to produce xylem and phloem , which are tissues these tissues can then group together to produce organs hence the plant can grow


Name a plant tissue



Name some organs of a plant



Cloning plants

The presence of meristems (as sources of unspecialised cells ) allows a plant to be cloned from cuttings . A clone of a plant may be made to reproduce desirable qualities within a plant. A cutting is a section of plant in the meristems that is cut from the plant and placed in soil in order to produce a clone of the plant this is possible as the meristem contains meristem cells which are unspecialised. This means an entire new plant can grow from the cutting which will be a clone of the parental plant.

If you stick cuttings in soil they will not always grow however if you add rooting powder which contains plant growth hormones (auxins) then the plant will grow as they will rapidly begin to grow roots .

Rooting powder helps growers produce lots of clones of a plant very quickly . Using cuttings is far faster than using seeds which is an advantage and there is a guaranteed chance that the plant will be identical to the one the grower wishes to replicate . This cannot be guaranteed by a seed as even within the same species there is variation.

Sometimes producing clones of plants is unsustainable as if there is a fault in the genes or they have a low resistance to a certain disease the entire plant culture could be quickly whiles out as the vunrablities of the parental plant is shared by all the clones



Some parts of plants respond to the environment when growing a response / sensitivity to light is known as phototropism . Shoots are positively phototropic they tend to grow towards the light , roots are negatively phototropic they tend to grow away from the light.

Positive phototropism - plants need sunlight for photosynthesis . Without light energy plants cannot photosynthesis and consequently do not produce glucose and do not have large food molecules to break down in order to release energy. Photosynthesis happens in the leaves this a shoot will tend to grow in the direction of the sun so it's leaves are at an angle to it in order to absorb the maximum amount of light energy possible , in order to allow the plant to bend toward the light in this way auxins which are plant growth hormone collect on the shaded side of the plant , the presence of these auxins elongates the cells at this site of the plant causing the curvature of the shoot in the direction of the light.

Negative phototropism - negative phototropism occurs in the roots of plants , plants need water and nutrients from the soil to grow , therefore they move away from the light and grow deeper down into the soil or into more moist areas of soil so they can attain more water and nutrients


How does phototropism help

Phototropism increases a plants chances of survival as it allows it to attain more light for the process of photosynthesis and also allows it to absorb more water and nutrients which it needs for growth


What are auxins

Auxins are plant growth hormones they are produced at the tips of roots and shoots and diffuse backward in order to trigger cell elongation . If auxins are spread out evenly across the top of. Shoot or root all cells will grow evenly hence the plant will grow straight


Why do some experiments on phototropism not accurately show how plants are effected by light in the outside environment

Other factors impact the growth of a plant in an external environment and these other factors may not be accounted for in a scientific investigation , plus the sun which is the light source for the plant is not in a fixed posturing through the duration of the day as a light would be in an experiment as the sun moves across the sky and is also absent at night for periods of multiple hours.