First 614 words of the document:
B5 Growth and Development Answers
1. DNA is a very large molecule shaped like a twisted ladder. The shape is a double helix. Long strands of DNA make up
chromosomes. These are found in the nucleus of a cell. DNA carries the genetic code which controls how an
2. Mitosis is a type of cell division. Mitosis occurs wherever more cells are needed. It
produces two new cells that are identical to each other, and to the parent cell. The
process of growth and division is called the cell cycle.
3. Organelles are the different parts of the cell's structure. They do different jobs
within the cell and work together to allow the cell to perform a specific function, e.g.
ribosomes and mitochondria.
4. When a cell enters the growth phase of the cycle the number of organelles increase
and the chromosomes are copied the two strands of each DNA molecule separate
and new strands from alongside them.
When a cells enters the division phase of the cycle the copies of the
chromosomes separate and the cell divides.
5. Mitosis occurs:
To replace old tissues
6. Mitosis produces new cells. Each new cell has identical sets of chromosomes as the
parent cell, the same number of chromosomes as the parent's cell and therefore the same genes as the parent cell.
7. You can see chromosomes by using a light microscope, but only in dividing cells. The DNA is too spread out in other
cells. After the chromosomes are copied, the DNA strands become shorter and
fatter. Then you can see them.
8. Some plants and animals produce asexually. They use mitosis to produce cells
for a new individual. This means that each of the individuals produced in
asexual reproduction is genetically identical to the parent, so it is a clone of its
9. Meiosis is a special form of cell division used to produce male and female
gametes so it only takes place in the testes and ovaries. A human body cell
contains 46 chromosomes arranged in 23 pairs. The gametes are sperm or
eggs, and only contain half as many chromosomes (23) as the parent cell. This
is why meiosis is sometimes called reduction division. Meiosis produces
gametes that are all different no two gametes have exactly the same
10. See diagram on right.
11. At fertilisation, the nuclei of the sperm and an egg join to form the zygote.
The zygote contains 23 pairs of chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes
from the egg, thereby creating the correct number of 46 chromosomes for all body cells. It also means the zygote
contains a complete set of chromosomes from each parent. The zygote then divides by mitosis to produce a cluster of
cells called an embryo. The embryo continues to develop by mitosis to eventually become an adult individual.
12. Meiosis and sexual reproduction produce variation between offspring and parents:
When the gametes fuse, genetic information from two individuals is combined
From each gene, just one of each parent's alleles is passed on
Each offspring can have a different combination of alleles from either parent
The offspring have different characteristics from each other.
13. Genes are present on the chromosomes in each cell nucleus. Genes control growth and development in organisms and
the development of different characteristics. Genes control characteristics by providing instructions for the
production of proteins. Each gene codes for one protein.
Other pages in this set
Here's a taster:
DNA is a chemical code, or set of instructions. Our bodies need proteins for growth and development, and the DNA
controls which proteins are made. The code consists of four different chemicals, or bases, that always pair up in the
T always pairs with A
C always pairs with G
The order of these pairs of bases along the DNA molecule codes for all the different proteins. A section of DNA that
codes for one particular protein is called a gene.…read more
Here's a taster:
Plants cells are different to animal cells in another way. Unspecialised stem cells in plants are grouped together in
structures called meristems. Cells produced by meristems ensure that plants continue to grow in height and width
throughout their life. Animals stop growing in size once they become adults. Plant meristems divide to produce cells
that increase the height of the plant, length of the roots and girth of the stem. They also produce cells that develop
into leaves and flowers.