Topic 4: Genetics 4.1 Chromosomes, Genes, Alleles and Mutations

4.1.1 State that eukaryotic chromosomes are made of DNA and protein Define gene, allele and genome Define gene mutation Explain the consequence of a base pair substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anemia

4.1.1 State that eukaryotic chromosomes are made of DNA and protein Chromosomes are: located inside the nucleus consists of a two-paired strand of DNA which is supported by proteins 50% of the chromosome is composed of protein a)some for support (histones) b)and a small percent are enzymes (DNA polymerase, helicase, ligase)

4.1.2 Define gene, allele and genome Define gene mutation Gene a specific length of DNA of the chromosome. It is an heritable factor that controls a specific characteristic of an organism. A gene occupies a position called a locus Allele the two forms of a gene Genome all the genetic information of an organism Gene pool all the genes carried by the members of a population Mutation a permanent change in the sequence of base pairs

4.1.4 Explain the consequence of a base pair substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anemia A change in just one nucleotide means a different amino will be produced during translation Sickle-cell anemia is an example of this Hemoglobin is composed of 4 polypeptide chains, two alpha and two beta When Thymine is replaced with Adenine, the codon changes from GAA (glutamic acid) to GUA (valine) This results in a slightly different shape of the protein which causes it to crystallize at low oxygen levels. course1.winona.edu

4.1.4 Explain the consequence of a base pair substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anemia Symptoms physical weakness, heart and kidney damage Advantage of being a sickle cell carrier is malaria prevention More common in malaria infected areas such as West Africa African Americans are more likely to carry the trait If both parents are carriers there is a 25% chance that their child will have the disease Parents can perform an amniocentesis to see if their child has the disease and may decide to end the pregnancy nhlbi.nih.gov anthro.palomar.edu

4.2.1 State that meiosis is a reduction division of a diploid nucleus to form four haploid nuclei The purpose of meiosis is to produce 4 haploid gametes (sex cells) Somatic cells (body cells) are diploid (2n) – they have two of each chromosomes Sex cells are haploid (n) – they have half the number of chromosomes than the somatic cells have Humans have 46 chromosomes (2n, where n=23) 23 from mom and 23 from dad Human sex cells have 23 chromosomes(n)

4.2.2 Define homologous chromosomes Homologous chromosomes are two chromosomes, one from each parent a)They are the same length b)Carry the same genes at the same loci c)They are the same shape d)Normally contain different alleles of the gene e)They form pairs with each other during cell division f)Similar banding patterns g)Centromere joins them in same location biology.iupui.edu

4.2.3 Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells Meiosis is divided into two stages: a)Meiosis I b)Meiosis II Both meiosis I and meiosis II are divided further into: a)Prophase b)Metaphase c)Anaphase d)Telophase See animation at:

4.2.3 Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells Interphase Before meiosis begins the chromosomes must replicate Occurs during S-phase of meiosis The two homologous chromosomes become a pair of sister chromatids joined at the centromere

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

4.2.4 Explain that non-disjunction can lead to changes in chromosomes number, illustrated by reference to Down Syndrome When a pair of chromatids fail to separate and move to the same pole or When homologous pairs move to the same pole This is called “nondisjunction” Can occur during anaphase I or anaphase II People with Down Syndrome have an extra chromosome #21, so they have 47 chromosomes instead of 46 (called trisomy 21)

4.2.4 Explain that non-disjunction can lead to changes in chromosomes number, illustrated by reference to Down Syndrome Reasons for Down Syndrome All the female gametes develop before birth and remain dormant in Prophase I The older the woman, the more chance her eggs will be exposed to potential damage (chemicals, radiation) This can result in an error during anaphase Can occur during meiosis I or meiosis II Older women have a greater chance of having a childwith Down Syndrome Animations: Nondisjunction in meiosis I Nondisjunction in meiosis II

4.2.5 State that in karyotyping chromosomes are arranged in pairs according to their size and structure Can be used to determine genetic disorders

4.2.6 State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities Two types of pre-natal screening are: 1.Chorionic villus sampling 2.Amniocentesis 1.Chorionic villus sampling can be done weeks into the pregnancy The procedure requires obtaining tissue from the placenta risk is about 1% daviddarling.info

4.2.6 State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities 1.Amniocentesis can be done during the 16th week take a sample of amniotic fluid karyotyping can be performed from the exfoliated cells of the embryo that are floating in the fluid The cells can be analyzed to detect chromosome abnormalities risk is about 0.5% daviddarling.info

4.2.7 Analyze a human karyotype to determine gender and whether non-disjunction has occurred

Describe the behavior of the chromosomes in the phases of meiosis Chromosomes behave in the following manner: 1.Interphase Chromosomes replicate forming two identical chromatids 2.Prophase I Homologous chromosomes pair up Crossing over occurs 3.Metaphase I Homologous pairs move to the equator 4.Anaphase I Homologous pairs are separated and move to the poles 5.Telophase I Chromosomes uncoil Chromosomes are reduced from diploid to haploid

Describe the behavior of the chromosomes in the phases of meiosis 1.Prophase II Chromosomes recoil 2.Metaphase II Chromosomes line up at the equator 3.Anaphase II Chromatids are moved to the poles 4.Telophase II Chromosomes uncoil

Outline the formation of chiasmata in the process of crossing over During prophase I all the chromatids of the homologous pairs get very close together during the process of synapsis The maternal (mom) and the paternal (dad) chromosome exchange genetic information by the process of crossing over The location on the chromosome where this occurs is called the chiasmata

Outline the formation of chiasmata in the process of crossing over This process is an important source of genetic variation Usually occurs at 2-3 sites on a chromosome The end result is a set of recombinant chromosomes (recombinant chromosomes - that have different genetic combinations than their parents) and a set of non-recombinant chromosomes (same as parent chromosomes) See animation:

Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I The genetic information of the four haploid gametes created during meiosis are significantly different from the autonomic cells The main reason for these differences are: 1.Random orientation (independent assortment) 2.Crossing over Random orientation The lining up of the pairs of chromosomes during metaphase I is a completelyrandom event. Either the maternal or the paternal chromosome can migrate to one pole. The combinations created by random orientation alone is more than 8million.(2 possibilities for 23 pairs of chromosomes or 2 23 ) course1.winona.edu

Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I Crossing over During prophase I, crossing over can occur at any point on a chromosome This means that the genes from one chromatid can be traded to another chromatid It is estimated that there are between 20,000 – 25,000 individual human genes So, there would essentially be unlimited possible combinations (2 alleles for 25,000 genes or 2 25,000 )

State Mendel's law of independent assortment (his 2nd law) Explain the relationship between Mendel's law of independent assortment and meiosis Mendel’s Law of Independent Assortment States: Two or more pairs of alleles segregate independently of each other as a result of meiosis (the genes cannot be on the same chromosome) During metaphase I, any pair of characteristics can combine with another pair