DL 3

To understand why people are so different, we need to go deep inside the cell to the chromosomes in the nucleus. It will be helpful to compare the process in  organisms that do not produce different offspring different to the parent (asexual reproduction) with the process in other organisms that produce offspring different to the parents (sexual reproduction).  

Notes: 

• the term offspring is used in genetics to describe the "children" because we are talking about a whole range of different living things, including plants, not just people.

• the presence of a- at the start of a word means the word is the opposite (eg symptomatic = having symptoms, asymptomatic means not having symptoms).

DNA stands for deoxyribonucleic acid, which is a long and complex molecule that contains the unique genetic code of each human and almost all other organisms, including some viruses. Nearly every cell in an organism has the same DNA. In the way that a computer code gives instructions to programs, DNA gives instructions - but these instructions are for making all the proteins in our bodies.  Where a computer code is based on different combinations of 0s and 1s, the information in DNA is stored as a code made up of different combinations of four chemical bases: adenine (A), cytosine (C), guanine (G) and thymine (T).  

DNA sequences form genes that are found on chromosomes in the nucleus of most cells (mature red blood, skin, hair, and nail cells  do not contain DNA). 

60% of our DNA is the same as the DNA in strawberries and other fruit!

Cells are limited in the size they can reach, so when a multi-cell organism grows, its individual cells do not grow bigger, it makes new cells.  When cells reproduce themselves, the DNA in the nucleus must replicate (make a copy of) itself.  

The following short video is just to show in simple form how that quite complex process happens.  It is not required learning (it is in the Y12 Biology course.)

1.3a) MITOSIS

Mitosis (pronounced my-tosis) is one of two processes where cells divide to form "daughter cells". 

Mitosis is the process by which: 

• cells make copies of themselves so the organism can grow

OR

• single-cell animals reproduce (produce new individuals) in asexual reproduction. 

Text Reference pages 12-16

1.3b) MEIOSIS

Meiosis (pronounced my-osis)  is the process of cell division with the only purpose that of sexual reproduction. Two sex cells combine in sexual reproduction, so if the sex cells were the same as all other cells - carrying the normal amount of chromosomes, then the "daughter" cell would have double the chromosome number, the next generation would have 4 times as much... and cells would burst! 

Meiosis reduces the chromosome number to one half, so that two sex cells combining result in just the normal numbes of chromosomes in every new generation.

Meiosis has some of the same steps as mitosis, with a few more added that reduce the chromosome number to half.

Text Reference pages 12-16

1.3c)  MITOSIS vs MEIOSIS

Each method of cell division has a purpose in reproduction. What is it, and does it give an advantage?