GENETICS TOPIC 4

GENETICS TOPIC 4

4.1 Chromosomes, genes, alleles and mutation

State that eukaryotic chromosomes are made of DNA and proteins

Define- gene, allele, and genome

Define mutation

Explain the consequences of a base substitution mutation in relation to the processes of transcription and translation

Chromosome Structure During Interphase of the cell cycle chromosomes are in the form of long, thin threads, which cannot be seen with a simple microscope

As the nucleus prepares to divide, these threads undergo coiling and become much shorter and thicker.

Chromosomes of eukaryotic cells are associated with proteins. Each chromosome contains a single molecule of DNA along with associated proteins. Some of these proteins are structural and others regulate activities of the DNA

Chromosomes, genes, and mutations

The DNA molecule consists of a linear sequence of nucleotides. The linear sequences of the bases contains the genetic code in the form of triplet bases (codons)

Gene

A gene is a heritable factor that controls a specific characteristic. It is particular section of a DNA strand that, when transcribed and translated forms a specific polypeptide.http://www.youtube.com/watch?v=eOvMNOMRRm8

A triplet of bases in the DNA molecule is transcribed into a triplet of bases in the mRNA molecule, which is then translated into a specific amino acid.

AlleleAllele- a specific form of a gene occupying the same position as other alleles of that gene, but differing from other alleles by small differences in its base sequence

GenomeGenome- the whole genetic information of an organism

MutationsThe process of DNA replication is complex and mistakes sometimes occur- a nucleotide may be left out, an extra one may be added, or the wrong one inserted.

Gene mutation- a change in the sequence of the bases in a gene.

The insertion of an incorrect nucleotide is called a base substitution mutation.

When DNA containing an incorrect nucleotide is transcribed and translated, errors may occur in the polypeptide produced.

http://www.youtube.com/watch?v=5raJePXu0OQ&feature=related

This table shows the amino acids that are specified by different mRNA codons.

Most amino acids are coded for by more than one codon and so many substitution mutations have no effect on the final polypeptide. A mutation in the DNA triplet CCA into CCG would change the codon in the mRNA from GGU to GGC, but would still result in the amino acid glycine

Some have serious effects

Sickle-Cell AnemiaSickle- cell anemia results from a single base substitution.

It’s a blood disorder in which red blood cells become sickle shaped and cannot carry oxygen properly.

Sickle-cell occurs most frequently in people with African ancestry- about 1% suffer from the condition and between 10%-40% are carriers.

http://www.youtube.com/watch?v=9UpwV1tdxcs

Sickle-cell anemia is due to a single base substitution mutation on one of the genes that makes hemoglobin, the oxygen-carrying pigment in red blood cells.

Hemoglobin is made up of four subunits. Two alpha chains and two beta chains. The beta chains are affected by sickle-cell mutation.

To form a normal beta- chain the triplet base pairing in DNA is:

-----G-----A-----G----- I I I-----C-----T------C-----

The C-T-C on the coding strand of the DNA is transcribed into the mRNA triplet G-A-G, which is in turn translated into glutamic acid in the polypeptide chain of the beta subunit

If the sickle-cell mutation occurs, the adenine base (A) is substituted for thymine (T) on the coding strand.

-----G-----T-----G----- I I I-----C-----A-----C-----

C-A-C on the coding strand of the DNA is now transcribed into the mRNA triplet G-U-G which is translated to give the amino acid Valine. Valine replaces Glutamic Acid in the beta-chain.

Valine has different properties from glutamic acid and so this single change in the amino acid sequence has very serious side effects.

The hemoglobin molecule is a different shape, it is less soluble and when in low oxygen concentrations it deforms the red blood cell to give it the sickle shape.

Sickle cells carry less oxygen, which result in anemia. They are also rapidly removed from circulation leading to a lack of red blood cells which can lead to jaundice, kidney problems and enlargement of the spleen.

4.2 Meiosis

http://www.youtube.com/watch?v=lJzZ7p-47P8&feature=related

•State that meiosis is a reduction division of a diploid nucleus to form a haploid nulcei.

•Define homologous chromosomes

•Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which reults in four haploid cells.

•Explain non-disjunction, and how it leads to Down’s Syndrome

•State that karyotyping , chromosomes are arranged in pairs according to their size and structure

•State that karyotyping is performed using cells collected by chorionic villus sampling or amnioccentesis, for prenatal diagnosis of chromosome abnormalities.

•Analyze a human karyotype to determine gender and whether non-disjunction occurred.