Chromosomes + cell division

Genome

the entire DNA of an organism (the amount of DNA present in a diploid cell)

Modern molecular biology discovered that a big/small part of the genome consists of protein-coding genes

small
For example, in the human genome the genes coding for proteins only make up about 1.5% of the total DNA.

Diagram of our genetic makeup

Chromosome

Threadlike structures of supercoiled DNA carrying genetic info in the form of genes

How many chromosomes do most human cells contain? What are pairs of matching chromosomes called?

46 chromosomes
Homologous pairs

Remember that the total number of chromosomes in each species differs. Dogs have 78, humans 46 , rice plants 24

What is the only moment chromosomes are visible? Where can we find chromosomes?

During cell division

In the nucleus of each cell

What are the cells that do not have 46 chromosomes? Why and how many do they have?

Red blood cells have no nucleus and so have no chromosomes
Sex cells (called gametes) have half (23) because of meiosis

Key point
What are the sex chromosomes?

X and Y chromosomes

How are the chromosomes in a female and a male going to differ?

Each cell from a male has 22 pairs of chromosomes and 2 that do not form a pair -- X and Y chromosomes (46 total). Except the sperm

Each cell from a female has 23 matching pairs including a pair of X chromosomes (46 total). Except the egg cell

One of each of the 22 homologous pairs are shown, along with the X and Y sex chromosomes

How many chromosomes do human body cells have (somatic cells)? What name is given to these cells with these number of chromosomes?

46 in total (23 homologous pairs) — diploid

What cells are the exception to the number of chromosomes? How many do humans have? What is this called?

Gametes (sex cells)

23 chromosomes in total (one copy of each homologous chromosome)

This applies to most other organisms as well like dogs (diploid number of chromosomes is 78 whereas they have 39 haploid chromosomes)

What is mitosis?

A type of nuclear division that gives rise to two daughter cells that are genetically identical to the parent cell and are diploid cells

When does mitosis occur?

• Growth: mitosis produces new cells

• Repair: to replace damaged or dead cells

  • Need to know but is not on spec: replacing cells e.g skin cells and red blood cells

• Asexual reproduction: mitosis produces offspring that are genetically identical to the parent

• Cloning

All the cells in our bodies (except the gametes) are formed by mitosis from the ____. They all therefore contain the copies of all the chromosomes and genes of that

zygote

Describe the process of mitosis

• Just before mitosis, each chromosome in the nucleus copies itself exactly (forms X shaped chromosomes called chromatids)

• Chromatids line up along the centre of the cell where fibres (spindle) pull them apart

• Two new nuclei form at the poles of the cell and the cell divides into two; each new cell has a copy of each of the chromosomes

Why does each chromosome copies itself before the cell divides? What does this involve?

Each daughter cell will be able to receive a copy of each chromosome and be genetically identical to the parent cell. If not, both daughters cells won’t contain all the genes

This involves the DNA replicating and more proteins being added to the structure.

What is meiosis

It is a type of nuclear division that gives rise to 4 haploid daughter cells (these are the gametes) that are genetically different

Describe the process of meiosis

• The parent cell must copy each chromosome so that there is enough genetic material to be shared between 4 daughter cells

• First division:

  • chromosomes pair up in homologous pairs

  • Members of each homologous pair separated

  • Fibres pull them apart to form 2 nuclei with one chromosome pair

• Second division:

  • chromosomes will line up along the centre of these (plural) cells, and cell fibres will pull the two chromatids of each chromosome apart forming 4 haploid gametes 

• It must divide twice so that each of the daughter cells receives just one chromosome from each homologous pair

Is there genetic variation in meiosis? How? Why is this good?

Yes. The gametes formed by meiosis don’t all have the same combinations of alleles. During the two cell divisions of meiosis, the chromosomes of each homologous pair are shared between the two daughter cells independently of each of the other homololous pairs. This allows for much possible genetic variation in the daughter cells.

If there is a change in the environment the best adapted offspring survive and passe on alleles for survival

Descriibe how cell division by meiosis is different from cell division by mitosis (4)

For meiosis

• Produces 4 cells/has two cell divisions

• Porduces haploid cells

• Halves the chromosome number (ignore 23 chromosomes)

• Produces genetic variation/cells not genetically identical

• Produces gametes

• Takes place in gonads/ovaries/testes/sex organs

What is a similarity between mitosis and meiosis?

Chromosomes make a copy before cell division begins