CHAPTER 17 Molecular Genetics. Why do Animals look the same?

CHAPTER 17Molecular Genetics

Why do Animals look the same?

17.1 DNA

17.2 Genes

Chapter 17

Molecular Genetics

Learning Outcomes

After this section, you should be able to:

• describe the basic unit of DNA – the nucleotide;

• state and apply the rule of complementary base pairing.

17.1 DNA

A cell contains a nucleus and in the nucleus are chromatin strands that are made up of proteins and deoxyribonucleic acid (DNA).

an animal cell nucleus with chromatin strands inside

DNA molecule

protein molecules

17.1 DNA

How is DNA organised inside the cell nucleus?

Each DNA molecule consists of two parallel strands twisted around each other to form a double helix.

During cell division, the chromatin threads coil more tightly to form chromosomes inside the cell nucleus.

A molecule of DNA is wrapped around proteins to form a single chromatin thread.

17.1 DNA

What is DNA?

• A molecule that carries genetic information.

• It is made up of nucleotides.

• A nucleotide is made up of:

(1) a deoxyribose (sugar) molecule,

(2) a phosphate group, and

(3) a nitrogen-containing base

- adenine (A), thymine (T),

guanine (G), or cytosine (C).

17.1 DNA

Possible nucleotides that can be formed:

adenine nucleotide

thymine nucleotide

guanine nucleotide

cytosine nucleotide

bases

sugar-phosphate backbone

polynucleotide

These nucleotides can be joined together to form polynucleotides.

17.1 DNA

The DNA molecule is made of two anti-parallel polynucleotide strands. (The two strands run in opposite directions.)

The bases on one strand form bonds with the bases on the other strand according to the rule of base pairing.

17.1 DNA

Rule of base pairing

• Adenine (A) bonds with thymine (T)

• Guanine (G) bonds with cytosine (C)

These pairs of bases are called complementary bases.

• Adenine (A) and thymine (T) are complementary bases. • Guanine (G) and cytosine (C) are complementary bases.

17.1 DNA

The two anti-parallel strands of the DNA molecule coil to form a double helix structure.

bases

sugar-phosphate backbone

coiling of DNA

the double helix structure of DNA

17.1 DNA

Checkpoint

2. State the complementary base pair to the strand shown

below:

Answer:

1. DNA molecules found in the cell nucleus are compacted to form chromatin strands. Other than DNA, what other molecule is found in chromatin strands?

Answer: Protein molecules

17.1 DNA

3. State the ratio of:

(1) adenine : thymine, and

(2) guanine : cytosine

in the DNA of a cell.

Answer:

(1)1 : 1

(2)1 : 1

4. The strands in a DNA molecule are ___________. The strands coil together to form a ______ ______ structure.

anti-paralleldouble helix

17.1 DNA

REMINDER

• Nucleotide

17.1 DNA

17.2 Genes

Chapter 17

Molecular Genetics

Learning Outcomes

After this section, you should be able to:

• state that DNA molecules contain the genetic code;

• state what is meant by the genetic code;

• state that a gene is a specific sequence of nucleotides in a DNA molecule that controls the production of a polypeptide.

17.2 Genes

• The Eye Colour is Green due to proteins.

• Proteins are formed thanks to genes found in the DNA that encodes for eye colour.

• Over time, eye colour may change (either lighten or darken)

• This is due to different expressions of the gene which leads to different release of proteins thus a different colour!

What is a gene?

• It is a segment of DNA.

• The nucleotide sequence in the gene determines the protein formed thus the hereditary trait.

• Since there are four different nucleotides, for a gene made up of n nucleotides, there are 4n different combinations of nucleotides.

gene

DNA

polypeptide coded by the gene

17.2 Genes

• Three nucleotides in a gene form a codon and each codon codes for one amino acid.

• The genetic code states which amino acid each codon codes for.

Example:Codon

(DNA)

Amino acid coded for

TAC Methionine (M)

TAT Alanine (A)

CAT Lysine (K)

GAG Glutamic acid (E)

ACA Serine (S)

17.2 Genes

• How are proteins made?

• The DNA template is first transcribed into a messenger RNA (mRNA) molecule through a process called transcription.

• Transcription occurs in the nucleus of the cell. During transcription, the DNA codons in the gene are converted into RNA codons.

• The mRNA molecule is then translated into polypeptides through the process of translation.

• Translation occurs in the cytoplasm of the cell.

How are proteins made?

17.2 Genes

DNA template

transcription

mRNA - RNA contains

uracil (U) instead of thymine (T)

polypeptide

translation

17.2 Genes

It is a temporary molecule that is made when needed.

It is a permanent molecule in the nucleus.

It is a small soluble molecule.It is a large insoluble molecule.

No fixed ratio between A and U and between G and C.

Ratio of A:T and G:C is 1:1.

Nitrogen-containing bases are adenine (A), uracil (U), guanine (G) and cytosine (C).

Nitrogen-containing bases are adenine (A), thymine (T), guanine (G) and cytosine (C).

Sugar unit is ribose.Sugar unit is deoxyribose.

RNADNA (double helix)

DNA vs. RNA

17.2 Genes

17.2

Transcription and Translation

Genes

1

2

template strand gene unzips

transcription

ribosome

mRNA

mRNA molecule

3 attachment to ribosome

Translation

• Attachment of the mRNA to a ribosome is the start of the translation process.

• The codons on the mRNA tells the cell what amino acids to attach together and in what order.

peptide bondamino acids attached to tRNA

ribosome

codon

17.2 Genes

• Attachment of mRNA with ribosome• mRNA is then read by codons (every 3 bases)• Bases found on mRNA will be complementary to tRNA

(transfer) bases.• Thus, formation of polypeptide chain begins with

bonding of other tRNA (different amino acids).• Polypeptide chain elongates with reading of every 3

bases on mRNA by ribosome.• It will elongate till it reaches the termination codon

found on mRNA. • Once completed, ribosome shall release mRNA and

protein is released into cytoplasm for processing.

Chapter 17

Molecular Genetics