This document provides a concise overview of genetics, including DNA structure, protein synthesis, inheritance patterns, and genetic engineering. It highlights key concepts and modern applications with a focus on ethical issues. Ideal for a quick yet comprehensive understanding of genetics.
Life at molecular, cellular and tissue level
The code of life
- The nucleus is surrounded by a double nuclear membrane with pores. The pores form
the passage between the nucleus and cytoplasm of the cell
- The nuclear membrane encloses the nucleoplasm
- When the cell divides the chromatin network coils and thickens into separate, shorter
threat-like structure called chromosomes. The chromosomes carry genetic material in
the cell
Nucleic acids
Nucleic acids are living organic molecules that control the synthesis of protein in all living
cells by storing transferring genetic information
Two types of nucleic acids occurring in living cells:
1. DNA (deoxyribonucleic acid)
2. RNA (ribonucleic acid)
Location of DNA
- DNA mainly occurs in the nucleus where it forms part of the chromatin network, thus
it is known as chromosomal DNA
- Other DNA occurs in the mitochondria of plant and animal cells and in chloroplasts of
plant cells, this is known as extranuclear DNA
o DNA found in the mitochondria is known as mitochondrial DNA (mtDNA). It
passes from mother to child and is thus used to trace material lines through
generations
Chromosomes and genes
- Chromosomes are long, thin thread-like structures composed of DNA that is wrapped
around proteins called histones
- A short segment of a DNA molecule that codes for a particular protein is known as a
gene
- Each gene carries the code for synthesis if a particular protein, proteins determine
the characteristics of an organisms
- A gene is a segment of a DNA molecule that controls an inherited characteristic and
determines the appearance and functioning of an organism
- Chromosomes are only visible in dividing cells
Structure of DNA
- DNA is a giant molecule, consisting of two strands that are twisted to form a double
helix
- When unwound, it looks like a ladder. DNA is a polymer made up of a large number of
similar units called monomers
- The monomers of DNA are known as nucleotides
Nucleotides
Each nucleotide consists of three parts: >
->
Phosphate Ion
- Pentose sugar
- Phosphate ion
Nitrogenous
- Nitrogenous base
(guanine, cytosine, adenine, thymine or uracil) -
Base
Pentose Sugar
-
,Nitrogenous bases
- Adenine (A) and Guanine (G) are larger molecules known as purine bases
- Cytosine (C) and Thymine (T) are smaller molecules known as pyrimidine bases
Formation of a nucleotide
1. Deoxyribose combines with the phosphate ion
2. One nitrogenous base combines with deoxyribose
3. As there are four different nitrogenous bases, there are four different nucleotides
in a DNA molecule
Formation of DNA
- In the formation of DNA, the deoxyribose of one of the nucleotides forms a bond
with the phosphate group of another
- Two long strands, resembling the sides of a ladder are formed
- The sides of the DNA ladder consists of alternating deoxyribose molecules and
phosphate ions
- Each rung of the DNA ladder is formed by the linking of two nitrogenous bases; a
large purine and a smaller pyrimidine base
- The two bases together are known as a base pair (nitrogenous bases are joined by
weak hydrogen bonds, which are easily broken by enzyme action)
Nitrogenous bases combine as the follows:
- Adenine (A) to Thymine (T)
- Guanine (G) to Cytosine (C)
- Two hydrogen bonds from between adenine and thymine, and three hydrogen bonds
form between guanine and cytosine
- Each nucleotide. May be repeated in the DNA strand; the bases occur in any sequence
- The sequence of bases is of great importance, as it provides the code that gives the
instruction for the synthesis of proteins
- This is known as the 'genetic code' or the 'code of life'
- The sequence of bases in one DNA strand (template) always determines the sequence
in the other strand
- One DNA strand is the complement of another strand
Role of DNA
- DNA carries the genetic code for the synthesis of proteins. A gene is a short segment
of DNA with a specific sequence of nitrogenous bases
- This sequence of nitrogenous bases determines the sequence and the type of amino
acids that will combine to from a particular protein
- DNA can replicate to ensure that the genetic code is accurately transferred from one
generation to the next
Non-coding DNA
- Approximately 2% of DNA in living cells codes for protein. The rest of the DNA does
not carry information to produced proteins and is known as non-coding DNA
- Initially scientists incorrectly though that non-coding DNA has no function and
referred to it as 'junk DNA'
- Non-coding DNA varies considerably between individuals and is used in the DNA
fingerprinting/ profiling. Although scientists are still researching the importance of
non-coding DNA, certain function have already been confirmed:
o It plays an important role in the regulation and control of the expression of
genes in the coding DNA. It determines when and where genes are switched 'on'
and 'off'
o It also protects the genes from mutations and controls the process of copying
genes during transcription (protein synthesis)
, DNA replication
- Replication is the duplication of the DNA molecule to from two identical copies
- Replication takes place during interphase of the cells cycle
- The proteins (histones) which together, with DNA, form part of a chromosome also
duplicate during replication. Two identical units called chromatids are formed
- The two chromatids are held together by a centromere
Process of replication
- The double helix structure of the DNA molecule unwinds, and the two strands appear
in the shape of a ladder (with help from helicase)
- The weak hydrogen bonds break and the two DNA strands unzip
- Free-floating nucleotides in the nucleoplasm build up a complementary DNA strand
onto each of the original DNA strands
- Enzymes control the joining of nucleotides
- Each DNA strand serves as a template in which its complement forms
- Two identical copies are formed from the original DNA molecule, each compromising
one 'new' strand and one original strand
- The double stranded structure of DNA makes replication possible
Importance of DNA replication
1. During miotic cell division, one mother cell divides into two identical daughter cells
2. It is essential that DNA makes identical copies of itself before cell division to ensure
each daughter cell contains the same genetic information as the mother cell
3. Each daughter cell has identical DNA composition to the mother cell
Mitochondrial DNA (mtDNA)
- MtDNA occurs in the mitochondria and is not related to chromosomal DNA that occurs
in the nucleus
- It is shorter and circular in shape compared to chromosomal DNA
- The genes of mtDNA code for the enzyme that controls cellular respiration
MtDNA and relatedness
- Male sperm contain only 0, 1% of the number of mitochondria present in the female
ovum
- Most of these mitochondria occur at the base of the sperm tail which is discarded
when the sperm penetrates the ovum in fertilization
- The only mitochondria that remain in the zygote are those belonging to the ovum
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