Genetics

Learning objectives To describe the transmission of genetic

information. To define mitosis and meiosis. To differentiate mitosis and meiosis. To define dominant and recessive genes. To define alleles. To define genotype and phenotype. To differentiate between sex

chromosomes. To define sex-linked disease.

Genetics Gene =

Hereditary unit, a sequence of DNA

Genome = Entire set of genes

Gamete = Mature sex cell

Chromatin not dividing cell

Chromosome dividing cell

A chromosome

DNA Deoxyribonucleic acid The material for

genetic information Confined to the

nucleus Structure= sugar +

phosphate group +organic nitrogenous base ( A, G, C, T )

Genetic Code The base sequence

in DNA It determines the a

mino acid sequences in protein

3 consecutive bases (triple code) in the DNA Codon

Genetic codes

Protein synthesis The sequence of

bases in DNA genetic codes ~ genotypes

Genotypes determine the phenotype through production of enzyme.

One- gene- one enzyme hypothesis

RNA Single-stranded mo

lecule differ from DNA ( rib

ose instead of deoxyribose ; U instead of T)

mRNA, tRNA, rRNA

Protein synthesis

I. Synthesis of amino acids~ in mitochondria & chloroplast~ non-essential & essential amino acids2. Transcription ~ DNA strand base sequence of mRNA ~ mRNA directs the protein synthesis3. Translation ~ mRNA sequence of amino acid

Transcription

Chromosome Composed of DNA, protein & RNA Invisible in non- dividing cells chromatin shorten & intensifies in dividing cells 23 pairs in human Member of ear pair of chromosomes = hom

ologue Homologous pair = one from father, one fro

m mother Diploid (2N) vs Haploid (1N)

Chromosomes in cell division

DNA replicate

2 identical chains of DNA

The DNA are surrounded by protein coat, 2 identical strands ly

es side by side

the strands are attached by centromere

Cell cycle The sequence of events occurring bet

ween the formation of cell and its division into daughter cell.

Interphase ~ period of synthesis & growth, replication of DNA

Nuclear division ~ separation of chromatids

Cytoplasm division ~ division of cytoplasm

Mitosis The division of nucleus into daughter

nuclei containing identical sets of chromosomes to the parent cell.

cell numbers Growth, replacement, repairs cells,

asexual reproduction Interphase + Prophase + Metaphase

+ Anaphase + Telophase

Prophase

Early metaphase

Metaphase

Anaphase

Telophase

Cytokinesis

Cytokinesis The division of

cytoplasm1. Cell organelles become

evenly distributed2. Invagination of cell

membrane3. Continuous furrow

around the equator4. Complete separation into

2 cells

Significance of mitosis I Genetic stability~ same number of chromosomes of

parent & daughter cells~ same hereditary information in

parent & daughter cells~ no variation in genetic information Growth ~ number of cells

Significance of mitosis II Cell replacement~ mitosis produces new cells Asexual reproduction and

regeneration~ regeneration of missing parts e.g.

tail in lizard

Meiotic cell division Also call reduction division. Takes place in the reproductive tissue. Single duplication of chromosomes + 2 cycles of nuclear division & cytopla

smic division A single diploid cell gives rise to 4 hapl

oid cells.

The necessity for chromosome reduction In normal somatic cells, chromosomes are

homologous & diploid(2N). Gamete contains only one member of each

homologous pair and is haploid(1N) due to meiosis.

In sexual reproduction, the zygote after fertilization (sperm[1N] + ovum[1N]) is 2N.

So, meiosis will prevent the nuclear materials from doubling in amount in each new generation.

Process of meiosis First meiotic divisio

n: ~ interphase ~ prophase I ~ metaphase I ~ anaphase I ~ telophase

Secind meiotic division

~ interphase II ~ prophase II ~ metaphase II ~ anaphase II ~ telophase II

Crossing over

Significance of meiosis Conserve the number of

chromosomes in the cells of successive generation.

Random orientation of chromosomes

Crossing over of genetic material

Comparison between meiosis & mitosis Continuous, regular process One duplication of genetic material Involves separation of

chromosomes & other cell organelles.

Similar mechanism of cell division Involves increase in cell number.

Comparison between meiosis & mitosis A single division of chr

omosomes & nucleus. The number of chromo

mses remains the same(2N).

No crossing over. Daughter cells are iden

tical to parent cells 2 daughter cells are for

med.

A single division of chromosomes but a double division of nucleus.

The number of chromosomes is halved, 2N to 1N.

Crossing over present. Daughter cells are

genetically different from parental ones.

4 daughter cells are formed.

Cytokinesis Normally follows telophase and leads

into interphase. The cell membrane invaginate and ev

entually join up complete separation of the two cells

Inheritance

Terms to know Haploid Diploid Allele Homozygous

Heterozygous Genotype Phenotype Dominant Recessive

Father of Genetics:

Gregor Mendel

Back crossTT (homozygous dominant) x TT (homozygous

dominant)

TT (all are dominant)

Back crossTt (heterozygous dominant) x Tt (heterozygous dominant)

Dominant (TT, Tt) & Recessive (tt) in ration 3:1

Test crossTT (homozygous dominant) x tt (homozygous recessive)

Tt (all are dominant)

Test crossTt (heterozygous dominant) x tt (homozygous recessive)

Tt or tt (ratio is 1:1)

Sex chromosomes Each body cell: 22 pairs autosomes + 1 pair sex

chromosome Sex chromosomes: X & Y Except sex chromosomes, all homologous

pairs of autosomes are identical X- chromosome carries many genes while Y

chromosome carries fewer genes.

Sex chromosomes Female: ~ genotype: XX

(homogametic sex)

Male: ~ genotype: XY

(heterogametic sex)

Sex-linkage Characters controlled by the genes w

hich situated on the sex chromosomes, especially X, are called sex-linked characters.

For example ~ red-green colour blindness ~ Haemophilia

Genetic diagram: color blindness

Genetic diagram: color blindness

Carrier

Genetic diagram: Hemophilia

Multiple alleles Genes exists in more than 2 allelis forms in t

he same locus of given pair of homologous chromosomes.

Each allele produces a distinctive phenotype. For example: ANO blood group system ~ the human blood groups are controlled by

three alleles IA, IB, I

Crossing over During pairing up, the homologous ch

romosomes break and re-join with non-sister chromatid of its homologous member exchange genetic segments.

Mutation Sudden & stable inherited change of

the genetic material(DNA). Leads to differences among

individuals Provides raw materials for the

species of the organisms. Enhances natural selection. Takes place at any stage in the

development of all organisms.