How is Inheritance Explained - AOS2 - Chapter 7 & 8 - VCE Biology Flashcards

DNA stands for…

Deoxyribonucleic acid

Genetic material in prokaryotic cells

• Single chromosome

• Circular DNA & RNA

• Free-floating in the cytoplasm in an area called the nucleoid region

• Have plasmids (small rings of DNA)

Genetic Material in Eukaryotic Cells

• More than one chromosome (23 in humans)

• DNA in the form of condensed linear chromosomes

• Located within the nucleus of the cell

• DNA also located in mitochondria and chloroplasts

DNA is…

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

Structure of DNA as a polymer

Nucleic acid is a polymer composed of monomers called nucleotides

Structure of DNA Nucleotides

• Deoxyribose Sugar (Deoxygenised pentose sugar)

• Phosphate molecule

• 1 of 4 nitrogenous bases (Adenine, Thymine, Guanine and Cytosine)

Structure of RNA nucleotides

• Ribose sugar (Oxygenised pentose sugar)

• Phosphate molecule

• 1 of 4 nitrogenous bases (Adenine, Uracil, Cytosine and Guanine)

The alternating sugar and phosphate group in an RNA or DNA molecule form the…

Sugar-phosphate backbone

Purines

• Double-ringed nitrogenous bases

  Adenine, Guanine

Pyrimidines

• Single-ringed nitrogenous bases

  Thymine, Cytosine and Uracil (RNA)

Complementary Base Pairing in DNA

Adenine (A) → Thymine (T)

Cytosine (C) → Guanine (G)

Complementary Base Pairing in RNA

Adenine (A) → Uracil (U)

Cytosine (C) → Guanine (G)

DNA Molecule Structure

• DNA forms a double helix, with 2 antiparallel strands joined together by base pairing

• Weak hydrogen bonds form between the complementary nitrogenous bases

Charge of DNA

• Negatively charged due to the phosphate group and oxygen atoms

Chargaff’s Rule

That the DNA of any species contains equal amounts of Adenine & Thymine, and equal amounts of Guanine & Cytosine

Antiparallel Strands (5’ and 3’ ends)

• The end with the unlinked phosphate group is called the 5’ end

• The end with the unlinked sugar is called the 3’ end

Phosphodiester Bonds

• Occur as a result of condensation polymerisation reactions between the nucleotides

• Form between the sugar and phosphate of the 2 nucleotides

  (Bottom of the sugar and above the phosphate group)

  • Water is lost as a result of the condensation polymerisation reaction

Differences in RNA and DNA

DNA

• Composed of deoxyribose pentose sugars

• Double stranded molecule

• Has the nitrogenous bases Adenine, Thymine, Guanine and Cytosine

RNA

• Composed of ribose pentose sugars

• Single stranded molecule

• Has the nitrogenous base Uracil in the place of Thymine

Types of RNA

• mRNA (messenger RNA)

• tRNA (transfer RNA)

• rRNA (ribosomal RNA)

mRNA function

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

tRNA function

Delivers specific amino acids to the ribosome and recognises specific nucleotide sequences (codons) on mRNA strand that are complementary to its own anticodon.

rRNA function

Structural component of ribosomes within the cells

Genome

The sum total of an organism’s DNA, measured in the number of base pairs contained in a haploid set of chromosomes.

Ex. In the human genome there approximately 3 billion base pairs which are organised into 23 chromosomes

Genomics

The study of genes and their function

Gene

A section of DNA that codes for making a protein

(A specific DNA base sequence that codes for a specific protein)

Ex. Gene for ability to roll tongue

Allele

A different variation/version of the same gene

Ex. The gene for tongue rolling has 2 alleles

• R - can roll tongue

• r - cannot roll tongue

Links between genome, gene and allele

• The genome is the entire set of genetic information within an organism

• Within the genome, genes are specific sequences of DNA that code for specific proteins and determine traits.

• Each gene can have multiple alleles (variations) with slightly different nucleotide sequences which leads to different traits being expressed

Universality of DNA

• All living organisms inherit DNA from their parents

  (half from mother and half from father)

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

Heredity

The passing on of traits (via DNA) from parent to offspring

Genetics

The study of the mechanisms and patterns of inheritance through the passing of genetic information from one generation to the next

Chromatin and Chromosomes

When DNA tightly coils around histone proteins, chromatin is created

As chromatin continues to condense, chromosomes form

Parts of a chromosome

• Telomeres*

• Short arm

• Centromere*

• Long arm

• Sister chromatids*

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 on a chromosome

Chromatids

A single, individual, unduplicated chromosome

• Each chromatid can also be referred to as a chromosome

• A molecule of DNA

Sister Chromatids

Identical/duplicated chromosomes composed of the same sequence of DNA bases joined at the centromere (Have the same genes and alleles)

• Are not homologous to each other

• Can also be referred to as a chromosome

Homologous Chromosomes

Composed of one maternal and one paternal chromosome and have the same genes at the same gene loci

Non-homologous Chromosomes

Have different genes at the same gene loci

Ex. Chromosomes 8 and 16

Number of Chromosomes in Humans

• 46 (23 pairs of) chromosomes

Types of Chromosomes in Humans (Autosomes and Sex Chromosomes)

Autosomes (Pairs 1-22)

• Involved in cellular functions

Sex Chromosomes (Pair 23)

• Determines the biological sex of the organism

Somatic Cells

Cells of the body other than sex cells

• Ex. muscle cells, nerve cells, red blood cells etc.

• Contain 46 chromosomes (Diploid 2n - one chromosome inherited from each parent)

• Produced by mitosis

Gametes (Sex Cells)

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

• Ex. Ova in females and sperm in males

• Located/produced in the gonads

• Contain 23 chromosomes (haploid n)

• Produced by meiosis

Gonads

Organs that produce gametes from germline cells

• Males - sperm (gamete) produced in testes (gonad)

• Females - Ova (gamete) produced in ovaries (gonad)

Haploid

Describes a single set of chromosomes within a cell

• Noted by n = 23 in humans

• Gametes

Diploid

Describes 2 sets of chromosomes within a cell (one from each parent)

• Noted by 2n = 46 in humans

• 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 is not correlated to the complexity of the organism

Karyotype

An image of an organism's complete set of chromosomes in a cell, arranged in pairs from largest to smallest

Karyotypes show…

• The number of chromosomes in each cell

  • Determines species (23 pairs in humans)

• Genetic sex

  • Male (XY) or Female (XX）

• Chromosomal abnormalities (from extra or missing chromosomes)

  • Monosomy at chromosome pair 21 indicates Down’s Syndrome

How are homologous chromosomes matched/paired in a karyotype?

• Banding pattern

• Centromere position

• Length of chromatids

Indications of Females and Males on Sex Chromosomes

XX - female

XY - male

Aneuploidy

The addition or loss of an individual chromosome from a cell

Aneuploidy occurs as a result of…

Non-disjunction at anaphase stage in meiosis - failure to separate homologous chromosomes

Non-disjunction at Anaphase 1 results in…

• 2 gametes with a chromosome missing

• 2 gametes with an extra chromosomes

  No normal gametes are produced

Non-disjunction at Anaphase 2 results in…

• 1 gamete with a chromosome missing

• 1 gamete with an extra chromosome

• 2 normal gametes

Gametes with a chromosome missing are…

Infertile, and will therefore not result in a baby

Types of Aneuploidy

Monosomy

Only one copy of a particular chromosome

• One missing chromosome

• Represented as 2n-1

Trisomy

Three copies of a particular chromosome

• One extra chromosome

• Represented as 2n+1

Examples of Aneuploidy Chromosomal Abnormalities

• Down Syndrome (trisomy at pair 21)

• Patau’s Syndrome (trisomy at pair 13)

• Edwards Syndrome (trisomy at pair 18)

• Klinefelter Syndrome (XXY - Trisomy at sex chromosomes)

• Turner’s Syndrome (X only - Monosomy at sex chromosomes)

Monoploidy

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

Are not considered haploid because their chromosomes represent a single, complete and operational set

• Represented by n

• Found in; ants, bees, fungi and algae, some fish, reptiles and amphibians

Monoploidy Disadavantage

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

Polyploidy

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

• Ex. 3n, 4n, 5n, 6n, 7n etc.

• Lethal in humans

• Found in; flowering plants, ferns and green algae, some fungi, fish and amphibian species

Polyploidy Advantage

• More hardy, bigger fruit

• Deeper roots to make them more resistant to environmental conditions

Polyploidy Disadvantage

Organisms that have an odd number of chromosome sets (e.g. 3n, 5n, 7n, etc.) are unable to produce gametes and are therefore infertile.

Sexual Reproduction

• Involves the contribution of genetic material from 2 parents

• Creates genetic diversity

Why do gametes (sex cells) need to be haploid?

Gametes are haploid because when the sperm and egg fuse during fertilisation, it ensures that the resulting 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 known as…

Meiosis

Meiosis

A reduction division, involving a reduction in the amount of genetic material from diploid (2n) to haploid (n) which produces gametes

Meiosis produces…

4 genetically different haploid daughter cells which later develop into gametes from one parent cell

Resulting haploid cells (gametes) from meiosis then develop into… via….

Males - 4 Sperm via Spermatogenesis

Females - Ova via Oogenesis

In one full round of meiosis, how many sex cells are produced?

Males - 4 sperm

Females - Only 1 viable egg and 3 polar bodies which later become absorbed

Germline cells

Cells that create gametes

• Diploid (2n)

Where does Meiosis occur?

In the gonads (sex organs)

• Testes (males)

• Ovaries (females)

How many cycles of cell division are there in meiosis?

2 cycles

• Meiosis 1

• Meiosis 2

How many round of DNA replication are there in Meiosis?

1 Round of DNA replication at Interphase

Stages of Meiosis 1

(Isabelle pushes malnourished adolescents to Chennai)

• Interphase

• Prophase 1

• Metaphase 1

• Anaphase 1

• Telophase 1

• Cytokinesis 1

At interphase, DNA is in the form of…

Loose uncoiled chromatin

Centrosome/ Pair of centrioles

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

Mitotic Spindle/Spindle Fibres

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

Interphase

Cell grows and undergoes normal biochemical processes

G1 - Cell growth

• Organelles replicate

• Proteins synthesised

S Phase - Synthesis

• DNA duplicates creating 92 sister chromatids from 46 sister chromatids

G2 Phase - Cont. Cell growth

• Continued synthesis of proteins for meiosis

Prophase 1

• Nuclear membrane and nucleolus break down

• Chromatin condenses and becomes visible, creating replicated chromosomes (2 chromatids) held by a centromere

• Spindle fibres begin to form from the centrioles at opposite poles of the cell

• Crossing-over of maternal and paternal alleles between homologous chromosomes occur during the latter part of this stage (creates genetic variation)

Crossing Over

The exchange of paternal and maternal alleles (genes) from non-sister chromatids/homologous chromosomes, creating recombinant chromosomes in daughter cells.

• Occurs at a position on the chromosome called the chiasma

  

Importance of Crossing Over

• Important source of genetic variation

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

Metaphase 1

• Homologous pairs of chromosomes line up in 2 files along the equator of the cell in random order (independent assortment - creates genetic variation)

• Spindle fibres attach to the centromere of each homologous pair

Independent Assortment

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

Importance of Independent Assortment

• Important source of genetic variation

The random alignment of homologous chromosomes give rise to different chromosome combinations in the resulting 4 gametes

Anaphase 1

• Spindle fibres contract, pulling the homologous chromosomes to opposite poles of the cell

• Recombinant sister chromatids held by the centromere remain intact

Telophase 1

• Spindle fibres disintegrate

• Two new nuclei form

• Nuclear membrane forms around chromosomes at opposite poles of the cell, creating two immediate daughter cells

• Cells begin to pinch in and cytoplasm divides as a cleavage furrow is formed for the cell to undergo cytokinesis

Cytokinesis 1

• Cells separate into 2 haploid daughter cells

• Chromosomes uncoil back into loose chromatin

Stages of Meiosis 2

(Isabelle Pushes Malnourished Adolescents to Chennai)

• Prophase 2

• Metaphase 2

• Anaphase 2

• Telophase 2

• Cytokinesis 2

Excludes Interphase and continues from Meiosis 1

Prophase 2

• NO DNA replication

• Chromatin re-condenses and becomes visible, forming 46 sister chromatids or 23 chromosomes

• Nuclear membrane and nucleolus break down

• Spindle fibres begin to form from the centrioles at opposite poles of the cell

Metaphase 2

• Homologous pairs of chromosomes line up in a single file along the equator of the cell in random orientation (independent assortment - creates genetic variation) across both cells

• Spindle fibres attach to the centromere of each chromosome

Anaphase 2

• Spindle fibres contract, splitting the recombinant sister chromatids at the centromere and pulling the chromatids to opposite poles of the cell

Telophase 2

• Spindle fibres disintegrate

• 4 new nuclei form

• Nuclear membranes form around the chromosomes at opposite poles of each cell, creating 4 immediate daughter cells

• Cells begin to pinch in and cytoplasm divides as a cleavage furrow is formed on both cells to undergo cytokinesis

Cytokinesis 2

• Each cell separates in half creating 4 haploid daughter cells

• Chromosomes uncoil back into loose chromatin

Similarities in Mitosis and Meiosis

• Occur in eukaryotic cells only

• Both begin with a diploid cell

• Have the same stages (interphase, prophase, metaphase, anaphase, telophase and cytokinesis)

• Use spindle fibres to separate chromsomes

Differences in Meiosis and Mitosis

Mitosis

• Purpose: Growth and repair

• Rounds of division: 1

• Number of daughter cells: 2

• Daughter cell Type: Identical to parent cell (Diploid 2n)

• Maintains chromosome number (2n → 2n)

• Occurs in somatic cells

Meiosis

• Purpose: Produce gametes for sexual reproduction

• Rounds of division: 2

• Number of daughter cells: 4

• Daughter Cell Type: Genetically unique to parent cell (Haploid n)

• Reduces Chromosome number (2n → n)

• Occurs in gonads

Crossing Over and Independent Assortment in Meiosis creates…

Infinite genetic variation in the haploid gametes produced

How do genes determine traits from the mother and father?

• One set of chromosomes is inherited from the mother and one set is inherited from the father

• These chromosomes contain genes which are inherited by the offspring

How are alleles (dominant and recessive) represented?

By a single letter

• Capital letter indicates the dominant allele

• Lowercase letter indicates the recessive allele

Ex.

The letter “A” represents petal colour

A - Dominant purple petals

a - Recessive white petals

Homozygote

When both the alleles inherited from each parent for a particular gene are the same