Biology EXAM UNIT 2, AOS 1,2,3

EXAM REVISIONNNNN

DNA stands for

Deoxyribonucleic acid

Genetic material in prokaryotic cells

Single chromosomes, circular DNA/RNA, free floating in the cytoplasm in an area called the nucleoid region, Have plasmids which are extra genes separate from the chromosomal DNA.

Genetic material in Eukaryotic cells

More than one chromosome (23 pairs in humans), the DNA is in the form of condensed linear chromosomes, it is located within the nucleus of the cell, the DNA is also located in mitochondria and chloroplasts in some plant and other animal species

DNA definition

An information molecule that encodes the instructions for the synthesis of proteins.

Structure of DNA as a POLYMER

Nucleic acid is the polymer of DNA and it is composed of monomers called nucleotides

Structure of DNA nucleotides

Deoxyribose sugar in the form of pentose sugars, phosphate molecule, 1 of 4 nitrogenous bases (A,T,G,C)

Structure of RNA nucleotides

Ribose sugar in the form of pentose sugars, phosphate molecule, 1 of 4 nitrogenous bases (A,U,G,C)

Sugar-Phosphate Backbone

The backbone of DNA and RNA that provide its shape formed from sugar and phosphate.

Purines

Double ringed nitrogenous bases (Adenine and Guanine)

Pyrimidines

Single ringed nitrogenous bases (Thymine, Cytosine and Uracil)

Complementary Base Pairing for DNA

Adenine with Thymine and Cytosine with Guanine

Complementary Base pairing for RNA

Adenine with Uracil and Cytosine with Guanine

DNA structure

Double helix shape with 2 antiparallel strands joined together by base pairings. Weak hydrogen bonds form between the complementary bases.

Charge of DNA

DNA is negatively charged due to the phosphate group and oxygen atoms

Phosphodiester Bonds

occurs as a result of condensation polymerization between the sugar and phosphate of the 2 nucleotides. Water is lost as a result of the reaction.

Antiparallel strands (5’ and 3’ ends)

the end with the unlinked phosphate is the 5’ end and the end with the unlinked sugar is the 3’ end

Differences in RNA and DNA

DNA is composed of deoxyribose sugar, double stranded, has Thymine as a nitrogenous base.

RNA is composed of ribose sugar, single stranded and has Uracil as a nitrogenous base.

Types of RNA

mRNA, tRNA and rRNA

mRNA function

Carries transcribed genetic material information from nucleus to the ribosomes for protein synthesis.

tRNA function

Delivers specific amino acids to the ribosomes and recognizes specific nucleotide sequence (codons) on mRNA that are complementary to its anticodon.

rRNA function

Structural component of ribosome within the cell

Genome

The sum total of an organism’s DNA.

Genomics

The study of genes and their function

Gene

A coding segment of DNA that is used for the creation of proteins.

Allele

A variation of a gene.

Link between genome, gene and allele

The genome is the entire set of genetic information within an organism and within the genome a gene is a specific sequence of DNA that codes for a protein. Each gene has multiple alleles which lead to the expression of different traits.

Why is DNA universal?

All living organisms inherit DNA from their parents

The structure of DNA is the same in all organisms only the sequence of nucleotides differs between species.

Heredity

The passing of traits from parent to offspring

Genetics

The study of the patterns pf inheritance through the passing of genetic information from generations.

Chromatin and Chromosomes

When DNA tightly coils around histone proteins, chromatin is formed. As chromatin continues to condense, chromosomes are formed.

Parts of a chromosomes label:

Telomeres, Short arm, Long arm, centromere, sister chromatids.

How many DNA molecules are in 2 sister chromatids

4 DNA molecules.

Telomeres

Repetitive base sequences at the end of every chromosome. Protects the ends of the chromosome from fusing with other chromosomes.

Centromere

The point at which 2 chromatids are held together

Chromatid

A single individual unduplicated strand of DNA

Sister Chromatids

Same genes and alleles held together at the centromere

How do we have two chromosomes each.

One is inherited from the mother and one is inherited from the father.

Homologous chromosomes

Composed of one maternal and one paternal chromosomes

They have the same genes and same gene loci but different alleles

Non-homologous chromosomes

Have different genes.

Chromosome number in Humans

46 (23 pairs)

Types of Chromosomes

Autosomes and Sex Chromosomes

Structural differences between prokaryotic and eukaryotic

Prokaryotic are less complex, no membrane bound organelles and are smaller in size. Eukaryotic are complex, have membrane bound organelles and are larger in size

Cell theory

All cells come from pre-existing cells

The cells are the smallest unit of life

All living things are composed of cells

Gene loci

A section on a chromosome where a specific gene is located

Chromosomes definition

Thread-like structures that contain the DNA of an organism.

How is DNA stored within the Nucleus of eukaryotes

It is in the form of chromosomes.

Autosomes

Autosomes are homologous chromosomes pairs 1-22.

Sex chromosomes

Non-Homologous pair 23

Somatic Cells definition, how many chromosomes and production method

Cells of the body other than sex cells

Contain 46 chromosomes

Produces by mitosis

Gametes (Sex cells) definition, how many chromosomes and production method

Reproductive cells that arise from germline cells and contain half of the genetic material of a somatic cell

23 chromosomes

Produced via meiosis by the gonads

Gonads in Males and Females

Organs that produce gametes from germline cells

Male- sperm produced by testes

Female- Ova produced by ovaries

Haploid and example

All of the genetic information contained in one complete set of an organism

Sex cell

Diploid Cell and example

All of the genetic information found in 2 sets of chromosomes within a cell

Somatic cells

Variability of chromosomes in terms of size and number in different organisms

Different organisms have different numbers of chromosomes and genes

Chromosomes can be different sizes

The number of chromosomes does not relate to the complexity of an organism

Karyotype

an image of an individuals complete set of chromosomes in a cell arranged from Largest to smallest

What do Karyotypes show?

Number of chromosomes

Biological sex

Chromosomal Abnormalities

How are homologous chromosomes sorted in a karyotype?

Banding Pattern

Centromere position

Chromatid length

Indication of Female and Male on Karyotype

Females have XX

Males have XY

What are the types of Variation in Chromosomes

Monoploidy

Polyploidy

Aneuploidy

Aneuploidy and give notation

The addition or loss of chromosomes

2n+1

2n-1

Anti codon

a sequence of three tRNA nucleotides

Codon

A sequence of three mRNA nucleotides

Aneuploidy occurs due to

Non-disjunction at anaphase and failure to separate homologous chromosomes

Non-disjunction at Anaphase 1 causes

• 2 gametes with a chromosome missing

• 2 gametes with an extra chromosome

• No normal gametes are produced

Non-disjunction at Anaphase 2 causes

1 gamete with a chromosome missing

1 gamete with an extra chromosome

2 normal gametes

Gametes with a chromosome missing are

Infertile and with therefore not result in a baby

Types of Aneuploidy

Monosomy and Trisomy

Monosomy

• One copy of a particular chromosome

• One missing chromosome

• 2n-1

Trisomy

Three copies of a particular chromosome

• One extra chromosome

• Represented as 2n+1

Examples of Aneuploidy

• Down syndrome (trisomy 21)

• Klinefelter’s syndrome (XXY)

• Turners syndrome ( X only)

Monoploidy

When the nucleus of a somatic cell of an organism only contains one set of chromosomes

Not considered haploid because their chromosomes represent a single complete and operational set

• Represented by n

• Found in ants, bees, fungi and algae

Monoploidy disadvantage

Any defective allele is the only copy for a particular gene, therefore diploid is an advantage.

Polyploidy

3 or more sets of an chromosomes in an organism’s nucleus

• Lethal in humans

• Found in plants, fungi and fish

Polyploidy advantage

• More hardy and bigger fruit

• Deeper roots to make them more resistant to environmental conditions

Polyploidy disadvantage

Organisms are unable to produce gametes and are infertile

Sexual Reproduction

Involves the contribution of genetic material from 2 parents

Creates genetic diversity

Why do gametes need to be haploid

Gametes are haploid because when the sperm and egg fuse during fertilization they ensure the zygote has 46 (23 pairs) of chromosomes and can therefore develop into a normal embryo/baby.

Production of gametes occurs through a type of cell division

Meiosis

Meiosis

A reduction division, involving a reduction in the amount of genetic material from diploid to haploid.

Meiosis produces

4 genetically different haploid daughter cells from one parent cell.

In Males meiosis creates

4 genetically different sperm (spermatogenesis)

In Females meiosis creates

1 viable egg (Ova via oogenesis) and 3 polar bodies which are absorbed back into the body.

Germline cells

Cells that create gametes

• Diploid (2n)

Where does Meiosis occur?

In the Gonads

How many cycles of cell divisions are there in meiosis

2 cycles

• Meiosis 1

• Meiosis 2

How many rounds of DNA replication are there is Meiosis?

1 round of DNA replication at Interphase

Stage of Meiosis 1

Interphase, Prophase, Metaphase, Anaphase, Telophase and Cytokinesis

Stage of Meiosis 2

Prophase, Metaphase, Anaphase, Telophase and Cytokinesis

Meiosis memory trick

(Isabel punches malnourished adolescents to Chennai)

At interphase DNA is in the form of

Lose uncolied chromatin

Centrosome/Pair of centrioles

A pair of barrel-shaped organelles located in the cytoplasm which are essential for cell division

Mitotic spindle

A structure composed of microtubules from centrioles which attach to centromeres of a chromosome and help split/pull them apart

Interphase

Cells grows and DNA is replicated from 46 to 92.

Prophase 1

• Chromatin condenses into chromosomes which become visible

• Spindle fibers form from the centrioles at opposite poles of the cell

• crossing over occurs

• Nuclear membrane begins to break down

Crossing Over

Crossing over is when the maternal and paternal homologous chromosomes exchange genetic material to create genetic variation. They create recombinant chromatids.

Occurs at chiasma

Importance of Crossing Over

Important source of genetic variation

All 4 daughter cells will have a different composition of alleles making each gamete genetically unique from each other

Metaphase 1

Homologous chromosomes align along the equator of the cell

Spidle fibers attach to the centromere of each homologous pair

This alignment is random, providing further genetic variation

This is called independent assortment

Independent Assortment

The random alignment of the homologous chromosomes along the equator of the cell

Importance of Independent Assortment

Important source of