Science - Growth and Development


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Growth and Development - B5

You begin life as a single cell.

Not only do you grow, but you develop in many ways throughout your life.

Muscle cell contract and relax to cause movement.

Nerve cells carry nerve impulses.

Newly formed human cells look much the same, then they develop into groups of specialized cells called tissues.

Tissues arrange themselves into organs, for example; the heart and brain in humans - and roots, leaves, and flowers in plants.

All the cells in your body come from just one orignal cell - a fertilized egg cell or zyote.


sperm + egg cell ----> fertilization ----> zyote (fertilized egg cell)

During the first week of growth, the zyote develops into a ball of about 100 cells. -->

The nucleus of each cell contains an exact copy of the original DNA. -->

Embryo grows --> some new cells become specialized and form tissues. -->

After two months --> main organs form --> developing baby is called a fetus. -->

A six-day embryo is made of about 50 cells.

You stop growing when your about 18-20 years old.

Flowering plants continue to grow throughout their lives:

- their stems frow taller.

- their roots grow longer.

- to hold themselves upright, most increase in girth or have some other means of support.

Plants increase in length by making new cells. These dividing cells are called meristem cells.

Cells in your body divide when you are growing.

Plant mertismem cells are unspecialized.

Animals also have spare back-up cells called stem cells. --> they divide, grow, and develop into any kind of cell the body needs.

Newt's can grow new legs or arms if they need to.

All cells start their lives with a nucleus.

Red blood cells carry oxygen.

Red blood cells develop cells from stem cells in your bone marrow. --> as they develop they make haemoglobin.

You have 23 chromosomes in your nuclei.

One set of 23 from mother, another 23 from father's nuclei.

When new body cells are made they contain the same cell parts called organelles.

Before a cell divides, it must gorw and make copies of;

- other organelles - such as ribosomes and mitochondria

- its nucleus, including the chromosomes

only then does the cells divide. this part of the process is called mitosis.

During mitosis, copies of chromosomes separate and the whole cell divides.


1. each chromosome has an identical copy attached to it. the membrane around the nucleus breaks down.

2. the chromosomes move to the centre of the cell.

3. the chromosome copies separate to opposite ends of the cell.

4. new nuclear membranes from before the cells divides.

Some plants and animals reproduce asexually. they use mitosis to produce cells fro a new individual.

--> meaning each of the individuals produced in asexual reproduction is genetically identical to the parent, so it is a clone of its parent.

most plants and animals reproduce sexually.

males and females make sex cells or gametes, which join up at fertilization.

a snail has both males and female sex organs.

male gametes are usually made in very large numbers.

they move to the female gametes by swimming or being carried by the wind or insect.

----> Sperm - 23 chromosomes -----> (egg - 23 chromosomes) >>>Fertilization>>> zygote - 46 chromosomes.

Meiosis halves the number of chromosomes in gametes. Fertilization restores the number in the zygote.


gametes are made by a special kind of division called meiosis.

In humans, meiosis makes gametes that:

- have 23 single chromosomes (one from each pair)

- are all different - no two gametes have exactly the same genetic information.

offspring from sexual reproduction are different from each other and from their parents. - we say they show genetic variation.


Meiosis starts with normal body cells. it only happens in sex cells.

(big cell) --> meiosis --> () () 4 copied cells () ()

^ the chromosomes in this diagram have been copied, the parent cell divides twice, producing four cells, there are four cells after meiosis. they have half the number of chromosomes as the parent cell.

there are four bases in DNA: adenine (A) thymine (T) guanine (G) and cytosine (C)

A always pairs with T

G always pairs with C

A+T are the same size as G+C

base pairing means that it is possible to make exact copies of DNA:

- weak bongs between the bases spilt, opening up the DNA from one end to form two strands.

- immediately, new strands start to from from free bases in the cell.

- as A always pairs with T, and G always pairs with C, the two new chains are identical to the original.

my body has more than 300 types of cells. - each cell has its own set of proteins.

some proteins make up the framework of cells and tissues. these are structual proteins.

if you took away all the water in an animals body, 90% of the rest is proteins.


keratin hair, nails, skin strong and insoluble

elastin skin springy

collagen skin, bone, tendons, legaments tough and not very stretchy

DNA is a cell's genetic code.

in a hair cell, the genes for the enzymes that make keratin will be switched on:

hair cell genes switched on -> enzymes for making keratin -> hair grows

but the genes for those that make amylase will be switched off.

In a salivary gland cell:

salivary gland cell genes switched on -> amylase secreted -> starch digested

some proteins are found in each type of cell, for example the enzymes needed for respiration. all cells respire, so the genes needed for respiration are switched on in all cells.

stem cells --> new skin - grown --> can be used for :

- the treatment of some dieseases

- the replacement of damaged tissue

scientists think the stem cells:

- turned into new blood vessels, or heart muscle cells

- made the heart tissue secrete chemicals that encourage growth of the patients own heart cells

Cells make proteins from about 20 different amino acids.

three bases on the DNA code for each amino acid. for example, TTT code for lysine.

the difference between DNA and mRNA are that mRNA has:

- only one strand

- the base U in place of the T in DNA

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