BASIC PRINCIPLES OF GENETICS

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Coiling of DNA1ry coiling of the duplex itself.2ry coiling around histone beads (nucleosome).3ry coiling of nucleosomes (chromatin fibers).4ry coiling forming loops.

Structure of DNATwo polynucleotide strandsTwo chains run in opposite directionSugar backbone Bases inside helix in Complementary base pairing (A=T & C=G)Strands bonded by weak hydrogen bonds

Definitions

Genome

the total number of genes

contained in one cell

Genotype

complete genetic constitution

of an individual

Phenotype

the physical appearance of

an individual that is

produced by the interaction

of his genes & environment

Trait

variation of an expressed

characteristic

Karyotype

systematic arrangement of

images of chromosomes

into homologous pairs

Autosomal chromosome

in humans, the 22 pairs of

chromosomes that are not

the sex chromosomes (XX or

XY)

Sex chromosomes

pair of chromosomes involved in sex determination; in males, the XY chromosomes; in females, the XX chromosomes

Allele

alternative forms of a gene that occupy a specific locus on a specific gene

Homozygous

having two identical alleles

for a given gene

Heterozygous

having two different alleles

for a given gene

Carrier

heterozygous individual who does not display symptoms of a recessive genetic disorder but can transmit the disorder to his or her offspring

Dominant

describes a trait that is expressed both in homozygous & heterozygous form

Dominant lethal

inheritance pattern in which individuals with one or two copies of a lethal allele do not survive in utero or have a shortened life span

Recessive

describes a trait that is only expressed in homozygous form and is masked in heterozygous form

Recessive lethal

inheritance pattern in which individuals with two copies of a lethal allele do not survive in utero or have a shortened life span

Incomplete dominance

pattern of inheritance in which a heterozygous genotype expresses a phenotype intermediate between dominant and recessive phenotypes

X-linked

pattern of inheritance in which an allele is carried on the X chromosome of the 23rd pair

X-linked dominant

pattern of dominant inheritance that corresponds to a gene on the X chromosome of the 23rd

pair

X-linked recessive

pattern of recessive inheritance that corresponds to a gene on the X chromosome of the 23rd

pair

Transcription

copying of DNA

sequences into mRNA

Translation

copying of mRNA

sequences into protein

Gene

Gene

the unit of heredity that occupies a specific locus on the chromosome. It is a DNA sequence that directs synthesis of a specific polypeptide chain

Genes

Structural

Regulatory

Operator

Structural Gene

directs synthesis of a

specific polypeptide chain

Regulatory Gene

repressor substance

secretion

Operator Gene

switches on structural

gene

The Genetic Code

This is the language of

mRNA

Based on the 4 bases of

mRNA

“Words” are 3 RNA

nucleotide sequences

called codons

Gene Mutation

Mutation

change in the nucleotide

sequence of DNA

A- Spontaneous Mutations(molecular decay)

B- Induced Mutations (mutagens)

- Chemicals

- Radiation

- Viral

Types of mutations

Types of Mutations

Chromosomal mutations involve changes in the whole chromosome.

Gene mutations result from changes in a single gene.

Effects of Gene Mutations

A- Neutral Effect (Silent mutation)

B- Harmful Effect

C- Beneficial Effect

Silent mutation

Another triplet for the

same AA

Harmful Effects

1.Errors in protein sequence partly or completely non-functioning proteins.

Harmful Effects

2.Germ cell mutations hereditary diseases/teratogenecity(heterozygous/homozygous)

Harmful Effects

3.

Somatic cell mutations

cancers

Regulation of gene expression

Not all genes are transcribed all the time.

Controlling mechanisms in the cell regulate the flow of enzymes & other proteins, determining which genes are turned on.

Some genes are needed for short periods of life cycle, others such as those involved in energy production may be operating all time.

DNA Functions

DNA functions

REPLICATION

METABOLISM

Replication

From one generation to another inheritance.

From cell to cell division.

Metabolism

gene expression & protein

synthesis (transcription &

translation)

Replication

DNA replication

it is the process by which

genetic information is

transmitted from one cell to

another

DNA replication

it starts by separation of the

2 strands. Each acts as a

template for a new strand

(semi-conservative)

DNA replication

DNA polymerase proceeds in

only one direction along the

helix (from the 5’ to 3’ end of

DNA) but starts in many sites.

Transcription

Transcription

the process whereby genetic information is transmitted from DNA to mRNA by RNA polymerase into complementary single-stranded copies of RNA

mRNA

are the processed transcripts of genes that code for polypeptides

Codon

a triplet of adjacent nucleotides in the mRNA chain that codes for a specific AA in the synthesis of a protein molecule

Exons

nucleotide sequences that is found in a gene, code information for protein synthesis, & is transcribed to mRNA

Introns

segments of a gene situated between exons that is removed before translation of mRNA & does not function in coding for protein synthesis

Promoterthe place where RNA polymerase 1st attaches. They vary from gene to another but contain sequences in common of 7 base sequence of all T’s & A’s

Termination

certain sequence that terminates gene transcription (downstream beyond the 5’ end)

Sense strand

the strand that is actually transcribed is the one with specific promoter sequence. The other non-sense strand is not used in transcription

ProcessingCAPPING: addition of a chemical cap to the 5’ end (attach RNA to the ribosome).SPLICING: excision of introns.POLYTAIL: addition of poly A tail (AAAA) to the 3’ end (it aids in transporting RNA molecule into the cytoplasm).

Translation

Translation

it is the process by which mRNA is translated into protein synthesis. It involves tRNA & rRNA

tRNA

one end binds a particular AA (all are 20), the other end has a 3-base sequence, the anticodon, that pairs with a particular codon on mRNA

rRNA

structural elements of ribosomes. They are the benches on which protein synthesis occurs

Steps of translation

mRNA moves along ribosomes

tRNA adapts

Starting codon= ATG

Stop codon= UGA, UAG, UAA

Summary

DNA transcription at chromosomes mRNA movement across NM rRNA translation at ribosomes protein

Patterns Of Inheritance

Patterns of inheritance

Autosomal Recessive Inheritance

Autosomal Dominant Inheritance

X-Linked Recessive Inheritance

X-Linked Dominant Inheritance

Autosomal Recessive Inheritance

Autosomal recessive

pattern of recessive

inheritance that corresponds

to a gene on one of the 22

autosomal chromosomes

Autosomal Dominant Inheritance

Autosomal dominant

pattern of dominant inheritance that corresponds to a gene on one of the 22 autosomal chromosomes

X-Linked Recessive Inheritance

X-Linked Dominant Inheritance

Referencescnx.org

Basics Of Human Genetics Prof. Dr. RandaYoussef Kasr El Aini (Presentation)

All about genes. Maha Adel Shaheen, MD Professor of Dermatology & Venereology AinShams University (Presentation)

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