Genes, Chromosomes

and DNA

Genes and alleles

• A gene is a section of DNA that carries the information for a particular trait (characteristic) eg. Eye colour, hair colour, production of a particular enzyme. Now a sequence of bases on a DNA molecule.

• Alleles are different forms of the same gene ie. The gene is eye colour, the alleles are blue, brown and green.

Chromosomes are long stretches of DNA, within which lie genes. A single chromosome may contain hundreds of genes.

Below are the locations of some known genes on human chromosomes:

Chromosome structure

• Chromosomes are made up of chromatin – a complex of DNA and protein.

• The protein that DNA forms a complex with is mainly histone protein. Histone may regulate DNA functioning in some way.

• DNA is coiled very tightly so it can fit inside the nucleus.

Structure continued:

• Just before cell division, the chromosomes double but remain joined at a region called the centromere.

• The two arms of the chromosome are called chromatids.

Chromosome replicated and prepared for cell division.

Chromosome in a non-dividing cell

Why are there two of each chromosome?

When cells in an organism divide, the new cells contain exactly the same number of chromosomes as the original cell. In a human cell there are 46. These are arranged in pairs. Why?

Because one chromosome in a pair comes from the female parent and one from the male parent.

Karyotypes

A karyotype is a picture of the chromosomes in an organism arranged in homologous pairs according to shape and size.

Karyotypes of humans

• The karyotype of a human male consists of 22 pairs of autosomes (44 chromosomes) plus a single X and a single Y chromosome.

• The karyotype of a human female consists of 22 pairs of autosomes (44 chromosomes) plus 2 X chromosomes.

Human Female Karyotype

Human Female: 44 + XX

Sex chromosomes: XX = female

Human Male Karyotype

Human Male: 44 + XY

Sex chromosomes: XY = male

Karyotypes

• Karyotypes are prepared from the nuclei of cultured white blood cells. The chromosomes in these cells are undergoing mitosis and when they reach metaphase a chemical is added to stop the cycle.

• The chromosomes are stained and a photo is taken of them under a microscope. The chromosomes are cut up and are arranged into homologous pairs according to shape and size.

• Do human karyotype activity on page 64. Note: there is an extra chromosome on purpose!

• Then do page 60-63

DNA – is a type of nucleic acid

There are two types of nucleic acid:1) DNA = deoxyribonucleic acid. The role of

DNA is to pass on genetic information from one generation to the next. It is the chemical that genes are made out of.

2) RNA = ribonucleic acid. RNA differs to DNA by the lack of one oxygen atom. RNA is involved in different cell processes such as protein synthesis.

Counting carbon atoms

• See examples on the board

DNA

• A DNA molecule is like a twisted ladder (the double helix). The 2 strands make up a DNA molecule which are the sides of the ladder. These are made up of a phosphate group alternating with a deoxyribose sugar.

DNA continued…

• Connecting the two strands, like the rungs of the ladder, are pairs of nitrogenous bases. There are 4 types of bases in DNA:

A= adenine T = thymineC= cytosine G= guanine

A only bonds with T and C only bonds with G. This is called the base-pairing rule.

Adenine and guanine are 2-ringed bases called purines

PurinesAdenine

• Double-ringed structures

Guanine• Always pair up with pyrimidines

Thymine and cytosine are one-ringed structures called pyrimidines

PyrimidinesCytosine

• Single-ringed structures

Thymine• Always pair up with purines

Uracil

How are the two strands held together?

• The 2 strands of the DNA ladder are held together by hydrogen bonds between the bases.

• Three hydrogen bonds form between guanine and cytosine

• Two hydrogen bonds form between adenine and thymine.

Nucleotides

• DNA is, therefore, made up of simple repeating units called nucleotides which consist of:

1) a phosphate group

2) a deoxyribose sugar

3) one of 4 nitrogenous bases (A,G, C, T)

Nucleotide

A DNA strand has a 5’ and a 3’ end

• See hand out: nucleic acids I:DNA.

• 5’ AGCTCCT TAGCTAA 3’

3’ TCGAGGAATCGATT 5’

RNA

• RNA exists as a single, not double stranded molecule.

• Like DNA it has a sugar group alternating with a phosphate group but the sugar is a ribose sugar.

• The bases of RNA can be adenine, cytosine, guanine or uracil. In RNA uracil replaces thymine.

Do creating a DNA model Page 72 –75

Do a DNA extraction