Unit 7: Genes and inheritance
Chromosomes Genes And Dna — HIA International - Grade 8 Revision Notes & Topical Questions
5 min read 9 parts Lesson 6 of 9 2 quizzes
Lesson summary
Chromosomes, genes and DNA — HIA INTERNATIONAL notes: Describe how chromosomes, genes and DNA are related and state the numbers of chromosomes in human cells.Chromosomes — thread-like structures in the nucleus of every cell.They are visible with a light microscope only when a cel…
- They are visible with a light microscope only when a cell is dividing
- Each species has its own number of chromosomes
- Chromosomes come in pairs
- Just before a cell divides
- The number of chromosomes does not show how complex an organism is
- Each chromosome is made of hundreds of genes
Section 1 of 9Chromosomes
Chromosomes
Chromosomes
- Chromosomes thread-like structures in the nucleus of every cell.
They are visible with a light microscope only when a cell is dividing
- They are visible with a light microscope only when a cell is dividing and they must be coloured with a special stain.
Each species has its own number of chromosomes
- Each species has its own number of chromosomes humans 46, fruit flies 8 and mango trees 40.
Chromosomes come in pairs
- Chromosomes come in pairs a human cell has two of each kind, so 23 pairs.
Chromosomes are numbered by length, so human chromosome 1 is the longest.
Just before a cell divides
- Just before a cell divides each chromosome copies itself and the two copies stay joined, giving a cross shape.
The number of chromosomes does not show how complex an organism is
- The number of chromosomes does not show how complex an organism is different species simply have different numbers.
Section 2 of 9Genes
Genes
Gene
Gene a short section of DNA on a chromosome that carries the instructions for one characteristic, by coding for a protein.
Each chromosome is made of hundreds of genes
- Each chromosome is made of hundreds of genes arranged in a fixed order along its length.
Human chromosome 1 has about 2000 genes and the much shorter chromosome 15 has about 600.
Alleles
Alleles different versions of the same gene.
Genes for eye colour are in the same place on every chromosome 15, but different alleles give blue or brown eyes.
Section 3 of 9DNA
1DNA (deoxyribonucleic acid)
- DNA (deoxyribonucleic acid) the chemical that chromosomes and genes are made of.
2Each chromosome is one enormously long DNA molecule
- Each chromosome is one enormously long DNA molecule so genes are also made of DNA.
3DNA has the shape of a twisted ladder called a double helix
- DNA has the shape of a twisted ladder called a double helix two strands twisted round each other and joined by pairs of bases.
4DNA molecules are too small to see
- DNA molecules are too small to see even with a microscope.
5The DNA in a cell
- The DNA in a cell contains a complete set of instructions to make a working cell and a whole organism.
6Every body cell carries the same DNA
- Every body cell carries the same DNA but different cells use different genes.
Section 4 of 9From cell to gene
1
A cell contains a nucleus.
2
The nucleus contains chromosomes, 23 pairs in humans.
3
Each chromosome is one long molecule of DNA.
4
DNA is a double helix of two strands.
5
A gene is a section of DNA that codes for one characteristic.
Section 5 of 9Chromosomes in body cells and sex cells
1Human body cells
- Human body cells contain 46 chromosomes in 23 pairs, one chromosome of each pair from each parent.
2Human sex cells (gametes)
- Human sex cells (gametes) contain 23 chromosomes, half the normal number, one from each pair.
3One pair are the sex chromosomes
- One pair are the sex chromosomes females are XX and males are XY.
4Red blood cells
- Red blood cells have no nucleus, so they contain no chromosomes.
Section 6 of 9Chromosome, gene and DNA compared
| Term | What it is | Where it is |
|---|---|---|
| Chromosome | One long coiled molecule of DNA | In the nucleus |
| DNA | The double helix molecule that carries the genetic code | Makes up chromosomes |
| Gene | A length of DNA that carries instructions for one characteristic | Along a chromosome |
| Allele | One version of a gene | At the same place on a pair of chromosomes |
Section 7 of 9How we learnt about DNA
1860s
- Gregor Mendel did experiments on the inheritance of characteristics in pea plants.
1902
- Walter Sutton and Theodor Boveri discovered that chromosomes are important in inheritance.
1920 to 1944
- Frederick Griffith and Oswald Avery showed that DNA is the genetic material.
1953
- Rosalind Franklin, Francis Crick and James Watson worked out the structure of DNA, with Maurice Wilkins.
2001
- 2001 the international Human Genome Project worked out the structure of all the DNA in human cells.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Chromosome | A thread-like structure of DNA in the nucleus that carries genes. |
| Gene | A section of DNA that codes for one characteristic. |
| DNA | The molecule that carries the genetic instructions of an organism. |
| Double helix | The twisted ladder shape of the DNA molecule. |
| Allele | A different version of the same gene. |
| Nucleus | The part of a cell that contains the chromosomes and controls its activities. |
| Sex chromosomes | The pair of chromosomes, XX or XY, that determine whether a person is female or male. |
| Genome | All the genetic material of an organism. |
Section 9 of 9Exam technique
1
Thinking genes and chromosomes are the same thing — a chromosome is one long DNA molecule that contains many genes.
2
Saying body cells have 23 chromosomes — body cells have 46 and gametes have 23.
3
Saying every cell has different DNA — all body cells carry the same DNA.
4
Thinking the cell nucleus and the atom nucleus are the same — they share a name but are different structures.
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